(VAR x y z)
(RULES
  +(x,0)  -> x
  +(x,s(y))  -> s(+(x,y))
  +(0,y)  -> y
  +(s(x),y) -> s(+(x,y))
  dbl(x) -> +(x,x)
  +(+(x,y),z) -> +(x,+(y,z))
  +(x,y) -> + (y,x)
)
(COMMENT R4 of \cite{AT2012})