* include/parallel/multiway_merge.h:
          (multiway_merge_*_unguarded):
          Pass sentinel directly, to allow correct determination.
          (multiway_merge_loser_tree_unguarded):
          Remove over-cautious assertion.
          (calls to multiway_merge_*_splitting):
          Parametrize with type that is correct in all cases.
        * include/parallel/losertree.h:
          (delete_min_insert (in many classes)):
          Correct and standardize assertions.

From-SVN: r134977