plusUDFM_C :: (elt -> elt -> elt) -> UniqDFM elt -> UniqDFM elt -> UniqDFM elt
plusUDFM_C f udfml@(UDFM _ i) udfmr@(UDFM _ j)
  -- we will use the upper bound on the tag as a proxy for the set size,
  -- to insert the smaller one into the bigger one
  | i > j = insertUDFMIntoLeft_C f udfml udfmr
  | otherwise = insertUDFMIntoLeft_C f udfmr udfml
plusUDFM_C :: (elt -> elt -> elt) -> UniqDFM elt -> UniqDFM elt -> UniqDFM elt
plusUDFM_C f udfml@(UDFM _ i) udfmr@(UDFM _ j)
  -- we will use the upper bound on the tag as a proxy for the set size,
  -- to insert the smaller one into the bigger one
  | i > j = insertUDFMIntoLeft_C f udfml udfmr
  | otherwise = insertUDFMIntoLeft_C f udfmr udfml
plusUDFM_C f udfml@(UDFM _ i) udfmr@(UDFM _ j)
  -- we will use the upper bound on the tag as a proxy for the set size,
  -- to insert the smaller one into the bigger one
  | i > j = insertUDFMIntoLeft_C f udfml udfmr
  | otherwise = insertUDFMIntoLeft_C f udfmr udfml