MAYBE

Problem:
 U11(tt(),M,N) -> U12(tt(),activate(M),activate(N))
 U12(tt(),M,N) -> s(plus(activate(N),activate(M)))
 U21(tt(),M,N) -> U22(tt(),activate(M),activate(N))
 U22(tt(),M,N) -> plus(x(activate(N),activate(M)),activate(N))
 plus(N,0()) -> N
 plus(N,s(M)) -> U11(tt(),M,N)
 x(N,0()) -> 0()
 x(N,s(M)) -> U21(tt(),M,N)
 activate(X) -> X

Proof:
 Open