(VAR x y z)
(RULES
  +(0,x) -> x
  +(x,0) -> x
  +(1,-(1)) -> 0
  +(-(1),1) -> 0
  -(0) -> 0
  -(-(x)) -> x
  -(+(x,y)) -> +(-(x),-(y))
  +(+(x,y),z) -> +(x,+(y,z))
  +(x,y) -> +(y,x)
)
(COMMENT from the collection of \cite{AT2012})