unionsWith package:nonempty-containers

unionsWith :: Foldable1 f => (a -> a -> a) -> f (NEIntMap a) -> NEIntMap a
nonempty-containers Data.IntMap.NonEmpty
The union of a non-empty list of maps, with a combining operation: (unionsWith f == foldl1 (unionWith f)). 
unionsWith (++) (fromList ((5, "a") :| [(3, "b")]) :| [fromList ((5, "A") :| [(7, "C")]), fromList ((5, "A3") :| [(3, "B3")])])
== fromList ((3, "bB3") :| [(5, "aAA3"), (7, "C")])
unionsWith :: (Foldable1 f, Ord k) => (a -> a -> a) -> f (NEMap k a) -> NEMap k a
nonempty-containers Data.Map.NonEmpty
The union of a non-empty list of maps, with a combining operation: (unionsWith f == foldl1 (unionWith f)). 
unionsWith (++) (fromList ((5, "a") :| [(3, "b")]) :| [fromList ((5, "A") :| [(7, "C")]), fromList ((5, "A3") :| [(3, "B3")])])
== fromList ((3, "bB3") :| [(5, "aAA3"), (7, "C")])