Simplify conclusion of a theorem once by conditional contextual rewriting.
DESCRIPTION
A call ONCE_SIMP_RULE thl (|- tm) will return |- tm' where tm' results
from applying the theorems in thl as (conditional) rewrite rules, as well as
built-in simplifications (see basic_rewrites and basic_convs). For more
details on this kind of conditional rewriting, see SIMP_CONV. The ONCE
prefix indicates that the first applicable terms in a toplevel term will be
simplified once only, though conditional subgoals generated will be simplified
repeatedly.
FAILURE CONDITIONS
Never fails, but may return the initial theorem unchanged.