(VAR x y z)
(RULES
   +(0,x) -> x
   +(1,-(1)) -> 0
   +(+(x,1),-(1)) -> x
   +(x,+(y,z)) -> +(+(x,y),z)
   +(+(x,y),z) -> +(x,+(y,z))
   +(x,y) -> +(y,x)
)
(COMMENT p. 1173 of JK86)