The effects.ruleset file contains all effects in play in a Freeciv scenario. 
They have the following form (this is perhaps the most complicated example I 
could find):

[effect_hydro_plant]
type  = "Output_Bonus"
value = 25
reqs  =
    { "type", "name", "range", "present", "quiet"
      "Building", "Factory", "City", TRUE, FALSE
      "Building", "Hydro Plant", "City", TRUE, FALSE
      "OutputType", "Shield", "Local", TRUE, TRUE
      "Building", "Hoover Dam", "Player", FALSE, FALSE
      "Building", "Nuclear Plant", "City", FALSE, FALSE
    }

The text in the brackets is the entry name, which just has to be unique, but
is otherwise not used. The 'type' tells Freeciv which effect type
is being added. If effect is active, 'value' gets added to the total of
the effect type's current value (i.e. values of individual effect entries
are cumulataive.) What that (total) value is actually used for depends
on effect type. The 'reqs' table contain a list of requirements for
this effect being in active. You need to satisfy all requirements
listed here for this effect to take effect in the game.
Requirements with present = TRUE must be present,
those with present = FALSE must not be present.

Value is integral amount parameter for many effects (must be in the range 
-32767 to 32767).

Requirement range may be one of: "None", "Local",
"CAdjacent" (Cardinally Adjacent), "Adjacent", "City", 
"Continent", "Player", "Allied, "World". Some requirement types may only work at 
certain ranges; this is not yet documented. In particular, at present, 
"Continent" effects can affect only cities and units in cities.

A requirement may have a 'survives' field, and if this is 'TRUE', the effect
survives destruction. This is supported for only a few conditions and
ranges: wonders (at world or player range), nations, and advances
(both at world range only).

A requirement may have a 'present' field, and if this is 'FALSE',
the requirement is negated (the condition must not be true for the req to be
met).

A requirement may have a 'quiet' field, and if this is 'TRUE', the help
system does not try to autogenerate text about that requirement. This
can be used if the help system's text is unclear or misleading, or if
you want to describe the requirement in your own words. The 'quiet'
field has no effect on the game rules.

Single requirement vector can have up to 40 requirements.


Requirement types and supported ranges
======================================

Tech:            World, Alliance, Team, Player
TechFlag:        World, Alliance, Team, Player
MinTechs:        World, Player
Achievement:     World, Alliance, Team, Player
Gov:             Player
Building:        World, Alliance, Team, Player, Continent, Traderoute, City, Local
BuildingGenus:   Local
Extra:           Local, Adjacent, CAdjacent, Traderoute, City
RoadFlag:        Local, Adjacent, CAdjacent, Traderoute, City
ExtraFlag:       Local, Adjacent, CAdjacent, Traderoute, City
Terrain:         Local, Adjacent, CAdjacent, Traderoute, City
Good:            City
UnitType:        Local
UnitFlag:        Local
UnitClass:       Local
UnitClassFlag:   Local
Nation:          World, Alliance, Team, Player
NationGroup:     World, Alliance, Team, Player
Nationality:     Traderoute, City
DiplRel:         World, Alliance, Team, Player, Local
DiplRelTile:     Alliance, Team, Player, Local
DiplRelTileOther:Local
DiplRelUnitAny:  Alliance, Team, Player, Local
DiplRelUnitAnyOther: Local
Action:          Local
OutputType:      Local
Specialist:      Local
MinYear:         World
MinCalFrag:      World
Topology:        World
ServerSetting:   World
Age (of unit):   Local
Age (of city):   City
Age (of player): Player
MinSize:         Traderoute, City
MinCulture:      World, Alliance, Team, Player, Traderoute, City
MinForeignPct:   Traderoute, City
AI:              Player
MaxUnitsOnTile:  Local, Adjacent, CAdjacent
TerrainClass:    Local, Adjacent, CAdjacent, Traderoute, City
TerrainFlag:     Local, Adjacent, CAdjacent, Traderoute, City
TerrainAlter:    Local
CityTile:        Local, Adjacent, CAdjacent
CityStatus:      Traderoute, City
Style:           Player
UnitState:       Local
Activity:        Local
MinMoveFrags:    Local
MinVeteran:      Local
MinHitPoints:    Local


MinSize is the minimum size of a city required.
AI is ai player difficulty level.
TerrainClass is either "Land" or "Oceanic".
TerrainAlter is "CanIrrigate", "CanMine", or "CanRoad".
CityTile is "Center" (city center), "Claimed" (tile owned) or
"Extras Owned" (extra on tile owned).
CityStatus is "OwnedByOriginal"
DiplRel is a diplomatic relationship.
MaxUnitsOnTile is about the number of units present on a tile.
UnitState is "Transported", "Transporting", "OnNativeTile", "OnLivableTile",
"InNativeExtra", "MovedThisTurn" or "HasHomeCity".
Activity is "Idle", "Pollution", "Mine", "Irrigate", "Fortified",
"Pillage", "Transform", "Fortifying", "Fallout",
"Base", "Road", "Convert", "Cultivate", or "Plant".
MinMoveFrags is the minimum move fragments the unit must have left.
MinCalFrag is the minimum sub-year division the calendar must have reached,
if enabled (see [calendar].fragments in game.ruleset).
Nationality is fulfilled by any citizens of the given nationality
present in the city.
ServerSetting is if a Boolean server setting is enabled. The setting must be
visible to all players and affect the game rules.

Effect types
============

Tech_Parasite
    Gain any advance known already by amount number of other teams,
if team_pooled_research is enabled, or amount number of other players
otherwise. Note that if you have two such effects, they combine into
one much worse effect (the number of players required to gain an advance
is increased).

Airlift
    Allow airlift to/from a city. The value tells how many units per
    turn can be airlifted, unless server setting 'airlifttingstyle' sets
    the number unlimited for either source or destination city. If airlifts
    are set to unlimited, they are enabled by any positive value of this
    effect.

Any_Government
    Allow changing to any form of government regardless of tech prerequisites.

Capital_City
    The city with positive value is a capital city. Player's city with highest
    Capital_City value (or random among those with equal positive value) is
    the primary capital. Cities with lesser positive value are secondary
    capitals.

Gov_Center
    The city with this effect is governmental center. Corruption and
    waste depends on distance to nearest such city.

Enable_Nuke
    Allows the production of nuclear weapons.

Enable_Space
    Allows the production of space components.

Specialist_Output
    Specify what outputs a specialist is producing. Should be used with an 
OutputType requirement.

Output_Bonus
    City production is increased by amount percent.

Output_Bonus_2
    City production is increased by amount percent after Output_Bonus, so is 
multiplicative with it.

Output_Add_Tile
    Add amount to each worked tile.

Output_Inc_Tile
    Add amount to each worked tile that already has at least
    output_granularity (from game.ruleset) output.

Output_Per_Tile
    Increase tile output by amount percent.

Output_Tile_Punish_Pct
    Reduce the output of a tile by amount percent. The number of units to
remove is rounded down. Applied after everything except a city center's
minimal output.

Output_Waste_Pct
    Reduce waste by amount percent.

Force_Content
    Make amount' unhappy citizens content. Applied after martial law and unit 
penalties.

Give_Imm_Tech
    Give amount techs immediately.

Conquest_Tech_Pct
    Percent chance that a player conquering a city learns a tech from the
    former owner.

Growth_Food
    Food left after cities grow or shrink is amount percent of the capacity of 
the city's foodbox. This also affects the 'aqueductloss' penalty.

Popcost_Free
    Ignore amount points of unit's population cost when shrinking the city that
    built it.

Have_Contacts
    If value > 0, gives contact to all the other players.

Have_Embassies
    If value > 0, gives an embassy with all the other players owner has ever
    had contact with.

Irrigation_Pct
    The tile gets value % of its terrain's irrigation_food_incr bonus.
(This is how irrigation-like extras have an effect.)

Min_HP_Pct
    The minimum HP (pct from full) units that have not moved during
    previous turn will have in the beginning of the new turn,
    after the HP_Regen effects have been applied.

Mining_Pct
    The tile gets value % of its terrain's mining_shield_incr bonus.
(This is how mine-like extras have an effect.)

Make_Content
    Make amount unhappy citizens content. Applied before martial law and unit 
penalties.

Make_Content_Mil
    Make amount unhappy citizens caused by units outside of a city content.

Make_Content_Mil_Per
    Make amount per unit of unhappy citizens caused by units outside of a city 
content.

Make_Happy
    Make amount citizens happy.

Enemy_Citizen_Unhappy_Pct
    There will be one extra unhappy citizen for each value/100 citizens
    of enemy nationality in the city.

No_Anarchy
    No period of anarchy between government changes. (This also neuters
the Has_Senate effect.)

Nuke_Proof
    City is nuke proof.

Pollu_Pop_Pct
    Increases pollution caused by each unit of population by amount
percent (adds to baseline of 100%, i.e. 1 pollution per citizen).

Pollu_Pop_Pct_2
    Increases pollution caused by each unit of population by amount
percent (adds to baseline of 100%, i.e. 1 pollution per citizen).
This factor is applied after Pollu_Pop_Pct, so is multiplicative with it.

Pollu_Prod_Pct
    Increases pollution caused by shields by amount percent.

Health_Pct
    Reduces possibility of illness (plague) in a city by amount percent.

Reveal_Cities
    Immediately make all cities known.

Reveal_Map
    Immediately make entire map known.

Border_Vision
    Give vision on all tiles within the player's borders. Happens during
    turn change.
    Does nothing unless the borders setting is set to "Enabled". You can
    lock it if border vision rules are important to your ruleset.

Incite_Cost_Pct
    Increases revolt cost by amount percent.

Unit_Bribe_Cost_Pct
    Increases unit bribe cost by amount percent. Requirements are from the
point of view of the target unit, not the briber.

Max_Stolen_Gold_Pm
    The upper limit on the permille of the players gold that may be
stolen by a unit doing the "Steal Gold" and the "Steal Gold Escape"
actions. Evaluated against the city stolen from.

Thiefs_Share_Pm
    The permille of the gold stolen by a unit doing the "Steal Gold" and the
"Steal Gold Escape" actions that is lost before it reaches the player
ordering it. Evaluated against the actor unit.

Maps_Stolen_Pct
    The probability (in percent) that the map of a tile is stolen in the
actions "Steal Maps" and "Steal Maps Escape".
DiplRel reqs are unit owner to city owner.
Requirements evaluated against tile or city not supported.
Default value: 100%

Heal_Unit_Pct
    The amount of the units full health the heal unit action will restore.
Will not "heal" the unit to more than 100% of its health.
Default value: 100%

Illegal_Action_Move_Cost
    The number of move fragments lost when the player tries to do an action
that turns out to be illegal. Only applied when the player wasn't aware that
the action was illegal and its illegality therefore reveals new information.

HP_Regen_2
    Like HP_Regen, except that these effects are applied after Min_HP_Pct effect,
    and unit's full HP is rounded up to nearest ten. So even if unit's full HP
    is less than ten, it would get rounded to ten, and HP_Regen_2 of at least
    10% always heals at least one HP.

Illegal_Action_HP_Cost
    The number of hit points lost when the player tries to do an action
that turns out to be illegal. Only applied when the player wasn't aware that
the action was illegal and its illegality therefore reveals new information.
Can kill the unit.
If the action always causes the actor unit to end up at the target tile two
consolation prizes are given. An area with the radius of the actor unit's
vision_radius_sq tiles is revealed. The player may also get contact with the
owners of units and cites adjacent to the target tile.

Action_Success_Actor_Move_Cost
    The number of move fragments lost when a unit successfully performs an
action. Evaluated and done after the action is successfully completed. Added
on top of any movement fragments the action itself subtracts.

Action_Success_Target_Move_Cost
    The number of move fragments subtracted from a unit when someone
successfully performs an action on it. Evaluated and done after the action
is successfully completed. Added on top of any movement fragments the
action itself subtracts. Only supported for actions that targets an
individual unit. (See doc/README.actions)

Casus_Belli_Caught
    Checked when a player is caught trying to do an action. Will cause an
incident with the intended victim player if the value is 1 or higher. The
incident gives the intended victim a casus belli against the actor player.
A value of 1000 or higher is international outrage. International outrage
gives every other player a casus belli against the actor.
This effect is supported only for following actions:
 Poison City, Poison City Escape, Spread Plague,
 Sabotage Unit, Sabotage Unit Escape, Bribe Unit, Spy Attack,
 Steal Tech, Steal Tech Escape Expected, Targeted Steal Tech,
 Targeted Steal Tech Escape Expected, Incite City, Incite City Escape,
 Sabotage City, Sabotage City Escape, Targeted Sabotage City,
 Targeted Sabotage City Escape, Sabotage City Production,
 Sabotage City Production Escape, Steal Gold, Steal Gold Escape,
 Steal Maps, Steal Maps Escape, Suitcase Nuke, Suitcase Nuke Escape,
 Explode Nuclear, Nuke City, Nuke Units,
 Paradrop Unit, Paradrop Unit Conquer, Paradrop Unit Frighten,
 Paradrop Unit Frighten Conquer, Paradrop Unit Enter,
 Paradrop Unit Enter Conquer

Casus_Belli_Success
    Checked when a player does an action to another player. Will cause an
incident with the intended victim player if the value is 1 or higher. The
incident gives the intended victim a casus belli against the actor player.
A value of 1000 or higher is international outrage. International outrage
gives every other player a casus belli against the actor.
This effect is supported only for the same actions as Casus_Belli_Caught, and
following actions that always success:
 Investigate City, Investigate City Spend Unit,
 Establish Embassy, Establish Embassy Stay,
 Capture Units, Expel Unit, Conquer Extras, Heal Unit,
 Found City, Join City, Bombard, Destroy City, Attack, Suicide Attack,
 Conquer City, Unit Move, Help Wonder, Disband Unit Recover,
 Establish Trade Route, Enter Marketplace, Pillage

Casus_Belli_Complete
    Checked when a player completes an action that takes several turns
against another player. Will cause an incident with the intended victim
player if the value is 1 or higher. The incident gives the intended victim
a casus belli against the actor player. A value of 1000 or higher is
international outrage. International outrage gives every other player a
casus belli against the actor.
"Pillage" is the only action currently supported.

Action_Odds_Pct
    Modifies the odds of an action being successful. Some actions have a
risk: the actor may get caught before they can perform it. This effect
modifies the actor's odds. A positive value helps them. A negative value
makes it more probable that they will get caught. Currently supports the
actions "Incite City", "Incite City Escape", "Steal Gold",
"Steal Gold Escape", "Steal Maps", "Steal Maps Escape", "Suitcase Nuke",
"Suitcase Nuke Escape", "Sabotage City", "Sabotage City Escape",
"Targeted Sabotage City", "Targeted Sabotage City Escape",
"Sabotage City Production", "Sabotage City Production Escape",
"Surgical Strike Building", "Surgical Strike Production",
"Steal Tech", "Steal Tech Escape Expected", "Targeted Steal Tech",
"Targeted Steal Tech Escape Expected", "Spread Plague", "Poison City" and
"Poison City Escape".

Size_Adj
    Increase maximum size of a city by amount.

Size_Unlimit
    Make the size of a city unlimited.

Unit_Slots
    Number of unit slots city can have units in. New units cannot be built,
    nor can homecity be changed so that maintained units would use more slots
    than this. Single unit does not necessarily use single slot - that's defined
    separately for each unit type.

SS_Structural, SS_Component and SS_Module
    A part of a spaceship; this is a "Local" ranged effect. It (for now) 
applies to improvements which cannot be built unless "Enable_Space" is felt. 
Buildings which have this effect should probably not be given any other 
effects.

Spy_Resistant
    In diplomatic combat defending diplomatic units will get an AMOUNT
percent bonus. All Spy_Resistant's are summed before being applied.

Building_Saboteur_Resistant
    If a spy specifies a target for sabotage, then she has an AMOUNT
percent chance to fail.

Stealings_Ignore
    When determining how difficult it is to steal a tech from enemy, AMOUNT
previous times tech has been stolen from the city is ignored. Negative amount
means that number of times tech has already been stolen from target city does
not affect current attempt at all. With this effect it's possible to allow
diplomats to steal tech multiple times from the same city, or make it easier
for spies.

Move_Bonus
    Add amount movement to units. Use UnitClass' requirement with range of 
'Local' to give it a specific class of units only.

Unit_No_Lose_Pop
    No population lost when a city's defender is lost.

Unit_Recover
    Units recover amount extra hitpoints per turn.

Upgrade_Unit
    Upgrade amount obsolete units per turn.

Upkeep_Free
    Improvements with amount or less upkeep cost become free to upkeep (others 
are unaffected).

Tech_Upkeep_Free
    If this value is greater than 0, the tech upkeep is reduced by this value.
    For tech upkeep style "Basic" this is total reduction, for tech upkeep
    style "Cities" this reduction is applied to every city.

No_Unhappy
    No citizens in the city are ever unhappy.

Veteran_Build
    Increases the veteran class of newly created units of this type. The
total amount determines the veteran class (clipped at the maximum for the
unit).

Veteran_Combat
    Increases the chance of units of this type becoming veteran after combat 
by amount percent.

Combat_Rounds
    Maximum number of rounds combat lasts. Unit is the attacker. Zero and
    negative values mean that combat continues until either side dies.

HP_Regen
    Units that do not move recover amount percentage of their full hitpoints 
per turn.

City_Vision_Radius_Sq
    Increase city vision radius in squared distance by amount tiles.
    There must be a base effect present in the ruleset;
    one with no requirements at all, i.e., always active

Unit_Vision_Radius_Sq
    Increase unit vision radius in squared distance by amount tiles.

Defend_Bonus
    Increases defensive bonuses of units. Any unit requirements on this effect
    will be applied to the _attacking_ unit type. Requirements about
    properties of a specific unit are ignored.
    Attackers with "BadWallAttacker" flag will have their firepower set to
    "combat_rules.low_firepower_badwallattacker" from game.ruleset.

Attack_Bonus
    Increases offensive bonuses of units. Unit requirements on this effect are
    the attacking unit type itself. Requirements about
    properties of a specific unit are ignored.

Fortify_Defense_Bonus
    Percentage defense bonus multiplicative with Defend_Bonus, usually given
    to fortified units. Unit requirements on this effect are the defending unit
    itself.

Gain_AI_Love
    Gain amount points of "AI love" with AI(s).

Turn_Years
    Year advances by AMOUNT each turn unless Slow_Down_Timeline causes it
    to be less.

Turn_Fragments
    Year fragments advance by AMOUNT each turn.

Slow_Down_Timeline
    Slow down the timeline based on the AMOUNT. If AMOUNT >= 3 the timeline 
will be max 1 year/turn; with AMOUNT == 2 it is max 2 years/turn;
with AMOUNT == 1 it is max 5 years/turn; with AMOUNT <= 0 the timeline is
unaffected. The effect will be ignored if game.spacerace isn't set.

Civil_War_Chance
    Base chance in per cent of a nation being split by civil war when its
capital is captured is increased by this amount. This percentage is in-
creased by 5 for each city in civil disorder and reduced by 5 for each one
celebrating.

City_Unhappy_Size
    The maximum number of citizens in each city that are naturally content;
in larger cities, new citizens above this limit start out unhappy. (Before
Empire_Size_Base/Step are applied.)

Empire_Size_Base
    Once your civilization has more cities than the value of this effect,
each city gets one more unhappy citizen. If the sum of this effect and
Empire_Size_Step is zero, there is no such penalty.

Empire_Size_Step
    After your civilization reaches Empire_Size_Base size, it gets one more 
unhappy citizen for each amount of cities it gets above that. Set to zero to 
disable. You can use Empire_Size_Step even if Empire_Size_Base is zero.

Max_Rates
    The maximum setting for each tax rate is amount.

Martial_Law_Each
    The amount of citizens pacified by each military unit giving martial law.

Martial_Law_Max
    The maximum amount of units that will give martial law in city.

Rapture_Grow
    Can rapture grow cities.

Revolution_Unhappiness
    If value is greater than zero, it tells how many turns citizens
    will tolerate city disorder before government falls. If value is
    zero, government never falls.

Has_Senate
    Has a senate that prevents declarations of war in most cases.

Inspire_Partisans
    Partisan units (defined in units.ruleset) may spring up when this player's 
cities are taken.

Happiness_To_Gold
    Make all Make_Content and Force_Content effects instead generate gold.

Max_Trade_Routes
    Number of trade routes that city can establish.
    This is forced on trade route creation only. Existing trade routes
    are never removed due to reduction of effect value. This is to
    avoid micro-management, need to create same trade routes again
    after their max number has been temporarily down.

Fanatics
    Units with "Fanatics" flag incur no upkeep.

No_Diplomacy
    Cannot use any diplomacy.

Not_Tech_Source
    Tech cannot be received from this player by any means.

Trade_Revenue_Bonus
    One time trade revenue bonus is multiplied by pow(2, amount/1000).
    The amount value is taken from the caravan's home city.

Traderoute_Pct
    Percentage bonus for trade from trade routes. This bonus applies after
    the value of the trade route is already calculated. It affects one end
    of the trade route only.

Unhappy_Factor
    Multiply unhappy unit upkeep by amount.

Upkeep_Factor
    Multiply unit upkeep by amount.

Unit_Upkeep_Free_Per_City
    In each city unit upkeep is deducted by this amount. As usual, you can use 
with OutputType requirement to specify which kind of upkeep this should be.

Output_Waste
    Base amount in percentage that each city has in waste. Waste can be used 
with any output type, use an OutputType requirement to specify which.

Output_Waste_By_Distance
    For each tile in real distance that a city is from nearest
Government Center, it gets amount / 100 of extra waste.

Output_Waste_By_Rel_Distance
    City gets extra waste based on distance to nearest Government Center, relative
    to world size. The amount of this extra waste is (distance * amount / 100 / max_distance)

Output_Penalty_Tile
    When a tile yields more output than amount, it gets a penalty of
    -1 x output_granularity (from game.ruleset), unless the city working
    it is celebrating.

Output_Inc_Tile_Celebrate
    Like Output_Inc_Tile, but only applies if city is also celebrating.

Upgrade_Price_Pct
    Increases unit upgrade cost by amount percent. This effect works at
    player level. You cannot adjust upgrade costs for certain unit type or
    for units upgraded in certain city.

Unit_Shield_Value_Pct
    Increase the unit's value in shields by amount percent. When this effect
is used to determine how many shields the player gets for the actions
"Disband Unit Recover" and "Help Wonder" it gets access to unit state. When it is
used to influence the gold cost of "Upgrade Unit" it only has access to unit
type.

Retire_Pct
    The chance that unit gets retired (removed) when turn changes.
    Retirement only happens if there are no enemy units or cities within
    a few tiles. (This exists mainly to implement barbarian behavior.)

Visible_Walls
    Instruct client to show specific buildings version of the city graphics.
    Zero or below are considered normal city graphics.

    On the client side:
    Tileset format supports up to 7 different wall graphics.
    Values above 7 are considered 7.
    Values up to 7, but above amount of tileset provided graphics,
    cause the default walls graphics to show.

Tech_Cost_Factor
    Factor for research costs.

Building_Build_Cost_Pct
    Percentage added to building building cost, in shields.
    This also affects buy cost, as it changes amount of shields to buy.
    This affects value of the already built buildings, when calculating
    city incite cost or city migration score.

Building_Buy_Cost_Pct
    Percentage added to building buy cost.
    The cost is calculated for the amount of shields needed.
    Note that Building_Build_Cost_Pct can change that amount of shields.

Unit_Build_Cost_Pct
    Percentage added to unit building cost, in shields.
    This also affects buy cost, as it changes amount of shields to buy.
    This affects value of the already built units, when calculating
    city incite cost.
    This does not affect unit upgrade price. Use Upgrade_Price_Pct for that.
    This does not affect the amount of shields unit provides when
    Helping Wonder. Use Unit_Shield_Value_Pct for that.

Unit_Buy_Cost_Pct
    Percentage added to unit buy cost.
    The cost is calculated for the amount of shields needed.
    Note that Unit_Build_Cost_Pct can change that amount of shields.

Shield2Gold_Factor
    Factor in percent for the conversion of unit shield upkeep to gold upkeep.
    A value of 200 would transfer 1 shield upkeep to 2 gold upkeep. The range
    of this effect must be player or world. Note that only units with the
    "Shield2Gold" flag will be affected by this.

Tile_Workable
    If value > 0, city can work target tile.

Migration_Pct
    Increase the calculated migration score for the a city by amount in
    percent.

City_Radius_Sq
    Increase the squared city radius by amount. Currently, this can only
    usefully have "MinSize", "Building", or "Tech" requirements.

City_Build_Slots
    Increase the number of units with no population cost a city can build in
    a turn if there are enough shields.

City_Image
    The index for the city image of the given city style.

Victory
    Positive value means that player wins the game.

Performance
    Value is how much performance type culture city produces.

History
    Value is how much history type (cumulative) culture city produces.

National_Performance
    Value is how much performance type culture, not tied to any specific city,
    nation produces.

National_History
    Value is how much history type (cumulative) culture, not tied to any any
    specific city, nation produces.

Infra_Points
    City increases owner's infra points by value each turn. If overall points
    are negative after all cities have been processed, they are set to 0.


Ruleset defined effects
=======================
The effects User_Effect_1, User_Effect_2, User_Effect_3 and User_Effect_4
are "blank". They have no meaning by them self. The ruleset does everything
they do.


Effects and Lua
===============
Lua has some integration with effects. The effects module allows you to
get an effect type's current value given that it is evaluated against the
entities you specify. Only a subset of the entities a hard coded effect can
be evaluated against is supported. An effect type's value is the sum of the
effects of that type, having requirement vectors fulfilled given the entities
you specified, from effects.ruleset.

Example
-------
Get the value of Tech_Parasite for the player
effects.player_bonus(find.player(0), "Tech_Parasite")


Details about requirement types
===============================

The DiplRel requirement type
----------------------------
Look for the diplomatic relationship "Never met", "War", "Cease-fire",
"Armistice", "Peace", "Alliance", "Team", "Gives shared vision",
"Receives shared vision", "Hosts embassy", "Has embassy",
"Hosts real embassy" (not from an effect), "Has real embassy",
"Has Casus Belli" (reason for war), "Provided Casus Belli" or "Foreign".

A DiplRel is considered fulfilled for the range
 * world if some player in the world has the specified diplomatic
   relationship to some other living player.
 * player if the player has the specified diplomatic relationship to some
   other living player.
 * local if the first player has the specified relationship to the second
   player. Example: When testing a build requirement for an extra the first
   player is the owner of the unit and the second player the owner of the
   terrain the extra is built on.

Only the exact relationship required fulfills it. Example: An alliance or
an armistice agreement won't fulfill a "Peace" requirement.

It is possible to create a requirement that in some situations won't have a
player to check. In those cases the requirement will always be considered
unfulfilled. This applies to both present and not present requirements. The
ranges Alliance, Team, Player and Local needs a player. The Local range also
needs the player the first player's relationship is to.

Example: The requirements below are about the relationship to the owner of a
tile. The table shows in what situations a requirement is fulfilled.

Requirement is                         fulfilled when the tile is
                                     | domestic | unclaimed | foreign
"DiplRel", "Foreign", "Local", TRUE  | no       | no        | yes
"DiplRel", "Foreign", "Local", FALSE | yes      | no        | no

The DiplRelTile requirement type
--------------------------------
Mostly like the DiplRel requirement type.
The player is the tile owner. The second player is DiplRel's player.
Doesn't have the World requirement range as that would be redundant.

The DiplRelTileOther requirement type
-------------------------------------
Mostly like the DiplRel requirement type.
The player is the tile owner. The second player is DiplRel's second player.
Only works in the Local requirement range as DiplRel and DiplRelTile covers
the rest.

The DiplRelUnitAny requirement type
-----------------------------------
Check if any unit at the tile has the specified diplomatic relationship.
Note that it says any - not each. It being true for one unit is enough.
The player is the owner of a unit at the tile.
The second player is DiplRel's player.
Doesn't have the World requirement range as that would be redundant.

The DiplRelUnitAnyOther requirement type
----------------------------------------
Check if any unit at the tile has the specified diplomatic relationship.
Note that it says any - not each. It being true for one unit is enough.
The player is the owner of a unit at the tile.
The second player is DiplRel's second player.
Only works in the Local requirement range as DiplRel and DiplRelUnitAny
covers the rest.

The MaxUnitsOnTile requirement type
-----------------------------------
Check the number of units present on a tile. Is true if no more than the
specified number of units are present on a single tile.

Hint: By using negation ("not present") it is possible to check if a tile
has more than the given numbers. It is possible to combine a negated and a
non negated requirement to specify a range.

The UnitState requirement type
------------------------------
"Transported" is fulfilled if the unit is transported by another unit.
"Transporting" is fulfilled if the unit is transporting another unit.
"OnNativeTile" is fulfilled if the unit is on a tile with native terrain or
               with a native Extra. Doesn't care about details like cities
               and safe tiles.
"OnLivableTile" is fulfilled if the unit is on a tile where it can exist
                outside of a transport.
"InNativeExtra" is fulfilled if the unit is on a tile with an extra native
                to it.
"MovedThisTurn" is fulfilled if the unit has moved this turn.
"HasHomeCity" is fulfilled if the unit has a home city.
