:: [(a, b)] -> [(a, [b])] -package:speculate

groupSort [(1,'t'),(3,'t'),(2,'e'),(2,'s')] == [(1,"t"),(2,"es"),(3,"t")]
\xs -> map fst (groupSort xs) == sort (nub (map fst xs))
\xs -> concatMap snd (groupSort xs) == map snd (sortOn fst xs)