This page contains proofs to some assertions that are left as exercises in [Combinator engine].
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I = S K K

To prove this, remember that
    S x y z = x z { y z }
and
    K x y = x
So what is S K K p?
    S K K p = K p { K p }
            = p
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B = S {K S} K

Remember, we want
    B x y z = x { y z }

So, let's see:
    S { K S } K x y z = K S x { K x } y z
                      = S { K x } y z
                      = K x z { y z }
                      = x { y z }                []
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