MAYBE

Problem:
 active(first(0(),X)) -> mark(nil())
 active(first(s(X),cons(Y,Z))) -> mark(cons(Y,first(X,Z)))
 active(from(X)) -> mark(cons(X,from(s(X))))
 mark(first(X1,X2)) -> active(first(mark(X1),mark(X2)))
 mark(0()) -> active(0())
 mark(nil()) -> active(nil())
 mark(s(X)) -> active(s(mark(X)))
 mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
 mark(from(X)) -> active(from(mark(X)))
 first(mark(X1),X2) -> first(X1,X2)
 first(X1,mark(X2)) -> first(X1,X2)
 first(active(X1),X2) -> first(X1,X2)
 first(X1,active(X2)) -> first(X1,X2)
 s(mark(X)) -> s(X)
 s(active(X)) -> s(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)
 from(mark(X)) -> from(X)
 from(active(X)) -> from(X)

Proof:
 Open