unionsWith (++) [(fromList [(5, "a"), (3, "b")]), (fromList [(5, "A"), (7, "C")]), (fromList [(5, "A3"), (3, "B3")])] == fromList [(3, "bB3"), (5, "aAA3"), (7, "C")]
unionsWith (++) [(fromList [(5, "a"), (3, "b")]), (fromList [(5, "A"), (7, "C")]), (fromList [(5, "A3"), (3, "B3")])] == fromList [(3, "bB3"), (5, "aAA3"), (7, "C")]
unionsWith (++) [(fromList [(5, "a"), (3, "b")]), (fromList [(5, "A"), (7, "C")]), (fromList [(5, "A3"), (3, "B3")])] == fromList [(3, "bB3"), (5, "aAA3"), (7, "C")]
unionsWith (++) [fromList [(1,'a',"b"),(1,'b',"c"),(2,'a',"b")], fromList [(1,'a',"c"),(2,'b',"d"),(3,'c',"e")]] === fromList [(1,'a',"bc"),(1,'b',"c"),(2,'a',"b"),(2,'b',"d"),(3,'c',"e")]
let f r c a a' = show r ++ ":" ++ show c ++ ":" ++ a ++ "|" ++ a' in do unionsWithKeys f [fromList [(1,'a',"b"),(1,'b',"c"),(2,'a',"b")], fromList [(1,'a',"c"),(2,'b',"d"),(3,'c',"e")]] === fromList [(1,'a',"1:'a':b|c"),(1,'b',"c"),(2,'a',"b"),(2,'b',"d"),(3,'c',"e")]