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_conv pat cnv ss adds the conversion
cnv to the simpset ss to yield a new simpset, restricting the initial
filtering of potential subterms to those matching pat.