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