(VAR x y z)
(RULES
   +(a,x) -> a
   +(b,g(a)) -> a
   +(0,x) -> x
   +(x,+(y,z)) -> +(+(x,y),z)
   +(+(x,y),z) -> +(x,+(y,z))
   +(x,y) -> +(y,x)
)
(COMMENT Example 4.1 of Winkler/Middeldorp RTA 2013)