Editing Surreal number (section)

==Games==
{{main|Combinatorial game theory}}

The definition of surreal numbers contained one restriction: each element of {{mvar|L}} must be strictly less than each element of {{mvar|R}}.  If this restriction is dropped we can generate a more general class known as '''games'''.  All games are constructed according to this rule:

; Construction rule : If {{mvar|L}} and {{mvar|R}} are two sets of games then {{math|{{mset| ''L'' {{!}} ''R'' }}}} is a game.

Addition, negation, and comparison are all defined the same way for both surreal numbers and games.

Every surreal number is a game, but not all games are surreal numbers, e.g. the game [[star (game theory)|{{math|{{mset| '''0''' {{!}} '''0''' }}}}]] is not a surreal number.  The class of games is more general than the surreals, and has a simpler definition, but lacks some of the nicer properties of surreal numbers. The class of surreal numbers forms a [[field (mathematics)|field]], but the class of games does not. The surreals have a [[total order]]: given any two surreals, they are either equal, or one is greater than the other. The games have only a [[partial order]]: there exist pairs of games that are neither equal, greater than, nor less than each other. Each surreal number is either positive, negative, or zero. Each game is either positive, negative, ''[[zero game|zero]]'', or ''[[fuzzy game|fuzzy]]'' (incomparable with zero, such as {{math|{{mset|1 {{!}} −1}}}}).

A move in a game involves the player whose move it is choosing a game from those available in {{mvar|L}} (for the left player) or {{mvar|R}} (for the right player) and then passing this chosen game to the other player.  A player who cannot move because the choice is from the empty set has lost. A positive game represents a win for the left player, a negative game for the right player, a zero game for the second player to move, and a [[fuzzy game]] for the first player to move.

If {{mvar|x}}, {{mvar|y}}, and {{mvar|z}} are surreals, and {{math|1=''x'' = ''y''}}, then {{math|1=''x'' ''z'' = ''y'' ''z''}}.  However, if {{mvar|x}}, {{mvar|y}}, and {{mvar|z}} are games, and {{math|1=''x'' = ''y''}}, then it is not always true that {{math|1=''x'' ''z'' = ''y'' ''z''}}. Note that&nbsp;"{{math|1==}}" here means equality, not identity.