Problem:
 +(x,0()) -> x
 +(x,s(y)) -> s(+(x,y))
 +(0(),y) -> y
 +(s(x),y) -> s(+(x,y))
 *(k(),0()) -> 0()
 *(k(),s(y)) -> +(k(),*(k(),y))
 +(x,y) -> +(y,x)
 +(+(x,y),z) -> +(x,+(y,z))

Proof:
 Open