cons :: a → c → c
consif :: b → a → c → c
false :: b
filter :: (a → b) → c → c
nil :: c
true :: b
consif(true, X, Y) → cons(X, Y)
consif(false, U, V) → V
filter(I, nil) → nil
filter(J, cons(X1, Y1)) → consif(J(X1), X1, filter(J, Y1))