* little-mler.sml (LittleMLer): Introduce the ``prefixer''
function and its variants, using the two strategy for combining
slists.

* sexp-expressions.sml (SExpressions.combine_slist_curried): Add
the curried strategy for combining two slists.
(SExpressions.combine_slist_staged): Add the staged strategy for
combining two slists.
(SExpressions.combine_sexp): Refactor the function in order to
emphasize the differences among the two curried vs staged
strategies for combining two slists.