X2126 : %THEOREM%X2126 is a theorem.
FORALL x(I) [P(OI) x EQUIV EXISTS y(I) P y] EQUIV .FORALL x P x EQUIV EXISTS y P y
You may not use ADVICE, but all other rules and commands are allowed.
Only book theorems may be ASSERTed as lemmas without proof.
For a complete list of book theorems, type PROBLEMS.
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