drop all but the indicated top-level hypotheses
Major Section: PROOF-CHECKER-COMMANDS
Example:
(RETAIN 2 3) -- keep the second and third hypotheses, and drop
the rest
General Form:
(retain &rest args)
Drop all top-level hypotheses except those with the indicated
indices.
There must be at least one argument, and all must be in range (i.e.
integers between one and the number of top-level hypotheses,
inclusive).