In their maximal generality, simplification operations in HOL Light (as invoked
by SIMP_TAC) are controlled by a `simpset', which may contain conditional and
unconditional rewrite rules, conversions and provers for conditions, as well as
a determination of how to use the prover on the conditions and how to process
theorems into rewrites. A call ss_of_thms thl ss processes the theorems thl
according to the rewrite maker in the simpset ss (normally mk_rewrites) and
adds them to the theorems in ss to yield a new simpset.