Section 1.14

(A PDF of the below is available here.)

"Proof: If  , the axioms (A) give

."

Here is how each step is arrived at:

"Take  in (a) to obtain (c)."

so (a) gives .

"Since , (c) (with  in place of ) gives (d)."

If, for some , we have , then by (c), .  Then substitution gives .  Then we have:

and by (a), .