MAYBE

Problem:
 active(from(X)) -> mark(cons(X,from(s(X))))
 active(length(nil())) -> mark(0())
 active(length(cons(X,Y))) -> mark(s(length1(Y)))
 active(length1(X)) -> mark(length(X))
 mark(from(X)) -> active(from(mark(X)))
 mark(cons(X1,X2)) -> active(cons(mark(X1),X2))
 mark(s(X)) -> active(s(mark(X)))
 mark(length(X)) -> active(length(X))
 mark(nil()) -> active(nil())
 mark(0()) -> active(0())
 mark(length1(X)) -> active(length1(X))
 from(mark(X)) -> from(X)
 from(active(X)) -> from(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)
 s(mark(X)) -> s(X)
 s(active(X)) -> s(X)
 length(mark(X)) -> length(X)
 length(active(X)) -> length(X)
 length1(mark(X)) -> length1(X)
 length1(active(X)) -> length1(X)

Proof:
 Open