5. (very minor) For mapping-split, I don't think I understand the
      motivation for returning five values (< <= = >= >) instead of
      just three (< = >).  I see the connection of the 5 values with
      the earlier 5 procedures, but the argument in favor of only 3 is
      that those three will be disjoint (and if the <= and >= ones are
      needed, then mapping-union can be conveniently used).