PURE_REWRITE_TAC : (thm list -> tactic)
PURE_REWRITE_TAC advances goals but solves them less frequently than REWRITE_TAC; to be precise, PURE_REWRITE_TAC only solves goals which are rewritten to "T" (i.e. TRUTH) without recourse to any other tautologies.
#PURE_REWRITE_TAC[]([],"b = T");; ([([], "b = T")], -) : subgoals #REWRITE_TAC[]([],"b = T");; ([([], "b")], -) : subgoals