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Proposition: The composition of two functions is also a function,.
forall f, g, A, B, C: f: A -> B /\ g: B -> C => f o g: A -> C.
Proof: see lecture notes (will be discussed later).
Proposition: direct characterization of function composition
forall f, g, A, B, C: f: A -> B /\ g: B -> C => forall x in A: (f o g)(x) = g(f(x)).