* list of replacements repls. This matching is in the sense of algebraic
* substitutions. Matching starts with pat.op(factor) of the pattern because
* the factors before this one have already been matched. The (possibly
* list of replacements repls. This matching is in the sense of algebraic
* substitutions. Matching starts with pat.op(factor) of the pattern because
* the factors before this one have already been matched. The (possibly