Stack 'em Diplo
===============

Stack 'em aims to eliminate all stalemate lines on all possible maps, with minimal additional complexity and minimal change to the way the game of Diplomacy is played. The spirit of the following rules is that multiple units of the same nationality may occupy the same space, but units of different nationality may not. As much as possible, multiple units in a single space behave the same way that separate units in separate spaces do. In particular, all units still have the same strength, and ONLY AN EXPLICIT SUPPORT ORDER CAN INCREASE THE STRENGTH OF AN ATTACK. 

1. If two or more units of the same nationality are ordered to move into the same space, then they will all occupy it, provding the moves are otherwise able to overcome foreign opposition. Units of the same nationality do not stand each other off.

2. Even when sharing a space, each unit is still separate and is ordered separately. Prefixing an order with a number, such as "2A BUR-MUN", is an abbreviation for ordering multiple units of the same type in the same space to do the same thing (and should not be thought of as ordering a double-strength unit around).

3. Two additional support orders are possible for every unit: to support a different unit out of the space it is in, or to support a (possibly foreign) unit into the space it is in. So, "A PAR-BUR, A PAR S A PAR-BUR, A BUR S A PAR-BUR" are valid orders to give to two armies in Paris and one in Burgundy. It is never necessary to order a unit to support a unit to hold in its own space, due to rule 14. 

4. If multiple units in one space attempt to move to another, they are all unsupported moves, and can all be stood off by a single unit. 

 Example 1:  F: 2A BUR-MUN.
             G: A MUN HOLD.
 Result: No movement.

 Example 2:  F: A BUR-MUN, A BUR S A BUR-MUN.
             G: A MUN HOLD.
 Result: The German army is displaced, and one French army advances (and the supporting unit stays behind).

5. The success of one move into a space does not automatically mean that all moves into the same space by the same nation succeed. Each move must overcome all opposition from other nations.

 Example 3:  F: A PAR-BUR, A PAR S A PAR-BUR, A MAR-BUR.
             G: A MUN-BUR.
 Result: One army from Paris advances. The other armies do not move. The army in Munich stands off the army in Marseilles.

 Example 4:  F: 2A PAR-BUR, A PAR S A PAR-BUR.
             G: A MUN-BUR.
 Result: One army from Paris advances. The other one is stood off by the army in Munich.

6. A dislodged unit may retreat to a space occupied by units of its own nationality, provided that space was not contested. Dislodged units of the same nationality can retreat to the same space without conflict. If dislodged units of different nationalities attempt to retreat to the same space, they are all destroyed. Even if one of the nations has numerical superiority, or occupies the space, they are still all unsupported retreats.

 Example 5:  F: A BUR-MUN, A RUH S A BUR-MUN, A BEL-RUH.
             G: A HOL-RUH, A KIE S A HOL-RUH, A MUN-BUR.
 Result: Only the army in Holland advances, dislodging the army in Ruhr, which can only retreat to Belgium. Burgundy is not open, because the army in Munich attempted to go there.

7. Support from a space for a move into itself is never cut.

 Example 6:  F: A PAR-BUR, A BUR S A PAR-BUR.
             G: A MUN-BUR, A RUH-BUR.
 Result: One army from Paris advances into Burgundy. The support from Burgundy itself is not cut.

 Example 7:  F: A PAR-BUR, A BUR S A PAR-BUR.
             G: A MUN-BUR, A RUH S A MUN-BUR.
 Result: No movement.

 Example 8:  F: A PAR-BUR, A BUR S A PAR-BUR.
             G: A MUN-BUR, 2A RUH S A MUN-BUR, A RUH-BUR, A RUH S A RUH-BUR.
 Result: One army from Munich advances, dislodging the army in Burgundy. No armies in Ruhr advance.

8. Two or more units of the same nationality can trade places without a convoy. They will not stand each other off.

9. While it is common, when writing support orders, to leave off the nationality or type of the unit being supported, or the coast it occupies, it becomes mandatory to include type in the case where different types of units occupy the same space and make the same move, and to include coast where two or more fleets occupy different coasts of the same space and make the same move. Ambiguity does not arise for support to hold, due to rule 14.

 Example 9:  F: F SPA(N)-MAO, F SPA(S)-MAO, F WMS S SPA-MAO
             E: F ECH-MAO
 Result: Nothing advances. It is impossible to accept this support order. The outcome would depend on which coast is being supported.

10. Each fleet may convoy one army. To convoy multiple armies through a sea space, there must be multiple fleets in that space which are ordered to convoy.

11. It is permissible to build on home centres occupied by domestic units. However only one unit can be built on any given home centre per year.

12. There is an ambiguity as to the distribution of support when multiple support orders are given to multiple units of the same type in the same place making the same move. The convention adopted is this: if not otherwise specified, all supports go to one of the units. Prefixing a number to the unit being supported to move is recognised as an abbreviation to distribute the supports accordingly. Ambiguity does not arise for support to hold, due to rule 14.

 Example 10: F: 2A PAR-BUR, 2A MAR S A PAR-BUR.
             G: A MUN-BUR
 Result: One army in Paris advances. It has both supports. The other is stood off by the army in Munich.

 Example 11: F: 2A PAR-BUR, 2A MAR S 2A PAR-BUR.
             G: A MUN-BUR
 Result: Two armies in Paris advance. They each have one support.

 Example 12: F: 2A PAR-BUR, 2A MAR S 2A PAR-BUR.
             G: A MUN-BUR, A RUH S A MUN-BUR.
 Result: Nothing advances. The two armies in Paris each have one support, as does the army in Munich.

 Example 13: F: 2A PAR-BUR, 2A MAR S A PAR-BUR.
             G: A MUN-BUR, A RUH S A MUN-BUR.
 Result: One army in Paris advances. It has both supports, and can overcome opposition from the army in Munich. 

13. When multiple units which share a space are given support orders, and the space is unsuccessfully attacked, then each attacking unit can cut one of the supports that it is normally eligible to cut. Where there is a choice, then the cuttable supports are cut in the order in which they were written. 

 Example 14: E: F NWG S NTH-NWY, F NWG S LON-NTH, F NTH-NWY, F LON-NTH.
             G: F DEN-NTH.
             R: F BAR-NWG, A NWY HOLD.
 Result: The fleet in London advances to the North Sea. The Russian fleet cuts the support for the move from the North Sea to Norway, since that support was written first.

 Example 15: E: F NWG S NTH-NWY, F NWG S LON-NTH, F NTH-NWY, F LON-NTH.
             G: F DEN-NTH.
             R: F BAR-NWG, F NWY-NWG.
 Result: Nothing advances. The fleet in Norway cuts the support for the English move to London, and the fleet in Denmark stands it off. The fleet in the Barents Sea cuts the support for the English move to Norway.

14. Although the strength of a move can only be increased by an explicit support order, defensive strength is implicitly cumulative among all units not moving, or failing to move out of the space. At least one of the units must be holding, supporting, or convoying, in order to receive additional support to hold from a neighbouring space. That is, for a move to be successful, it must have at least as many supports as there are valid, uncut supports from outside the space, plus units not moving, or failing to move out of the space. If successful, the move dislodges all units in the space. (This is a departure from behaving the same way separate units in separate space do. The excuse for this behaviour is that unlike selectively cutting supports, there cannot be selective dislodging of units, unless multiple nations are allowed to occupy a space at once. Allowing that would really complicate matters. If it is accepted that an attack must displace all or none, then the question becomes under what circumstances can a single attack displace multiple units. This answer simplifies a lot of things.)

 Example 16: E: F NWG S NTH-NWY, F NWG S LON-NTH, F NTH-NWY, F LON-NTH.
             G: F DEN-NTH.
             R: F BAR S F NWY-NWG, F NWY-NWG.
 Result: The fleet in the North Sea advances, dislodging the fleet in Norway. The fleet in Norway cuts support for the move from London to the North Sea, but despite the support of the fleet in the Barents Sea, it is not able to dislodge the two fleets in the Norwegian Sea.