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