cons :: b → c → c
false :: a
filter :: (b → a) → c → c
filtersub :: a → (b → a) → c → c
nil :: c
true :: a
filter(F, nil) → nil
filter(Z, cons(U, V)) → filtersub(Z(U), Z, cons(U, V))
filtersub(true, I, cons(P, X1)) → cons(P, filter(I, X1))
filtersub(false, Z1, cons(U1, V1)) → filter(Z1, V1)