Returns union of assumption lists of the given theorems.
DESCRIBE
When applied to a list of theorems, hyp_union returns the union (see
union) of their assumption lists. Straight repetitions only arise if there
were multiple instances of an assumption in a single assumption list. There is
no elimination of alpha-equivalent pairs of assumptions, only ones which are
actually equal.
hyp_union [A1 |- t1; ... ; An |- tn] = A1 u...u An
FAILURE
Never fails.
USES
Designed for internal use, in writing primitive inference rules.