Comment by jmj

S combinator always duplicates its last parameter, never deletes it. That's why K is needed for universality.

This can be proved by induction. Or you can cite Craig's theorem (the less known one) for that. See [1]

Honestly, I don't see the endgame here.

[1] https://math.stackexchange.com/questions/839926/is-there-a-p...