MAYBE

Problem:
 active(zeros()) -> mark(cons(0(),zeros()))
 active(tail(cons(X,XS))) -> mark(XS)
 mark(zeros()) -> active(zeros())
 mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
 mark(0()) -> active(0())
 mark(tail(X)) -> active(tail(mark(X)))
 cons(mark(X1),X2) -> cons(X1,X2)
 cons(X1,mark(X2)) -> cons(X1,X2)
 cons(active(X1),X2) -> cons(X1,X2)
 cons(X1,active(X2)) -> cons(X1,X2)
 tail(mark(X)) -> tail(X)
 tail(active(X)) -> tail(X)

Proof:
 Open