MAYBE

Problem:
 circ(cons(a,s),t) -> cons(msubst(a,t),circ(s,t))
 circ(cons(lift(),s),cons(a,t)) -> cons(a,circ(s,t))
 circ(cons(lift(),s),cons(lift(),t)) -> cons(lift(),circ(s,t))
 circ(circ(s,t),u) -> circ(s,circ(t,u))
 circ(s,id()) -> s
 circ(id(),s) -> s
 circ(cons(lift(),s),circ(cons(lift(),t),u)) -> circ(cons(lift(),circ(s,t)),u)
 subst(a,id()) -> a
 msubst(a,id()) -> a
 msubst(msubst(a,s),t) -> msubst(a,circ(s,t))

Proof:
 Open