MAYBE

Problem:
 q(q(x,y),z) -> q(x,q(y,z))
 q(x,q(y,z)) -> q(q(x,y),z)
 q(e(),x) -> x
 q(x,e()) -> x
 plus(x,plus(y,z)) -> plus(plus(x,y),z)
 plus(x,y) -> plus(y,x)
 plus(0(),x) -> x
 plus(s(x),y) -> s(plus(x,y))
 len(e()) -> 0()
 len(a()) -> s(0())
 len(q(a(),y)) -> plus(len(a()),len(y))

Proof:
 Open