(VAR x y z)
(RULES
   +(0,x) -> x
   +(-(x),x) -> 0
   +(+(x,y),z) -> +(x,+(y,z))
   +(x,y) -> +(y,x)
   -(0) -> 0
   -(-(x)) -> x
   -(+(x,y)) -> +(-(x),-(y))
)
(COMMENT Commutative Group Theory from \cite{PS81})