>>> singleton True [True]
>>> singleton [1, 2, 3] [[1,2,3]]
>>> singleton 'c' "c"
toLazyText (singleton c) = singleton c
singleton 1 'a' == fromList [(1, 'a')] size (singleton 1 'a') == 1
singleton 1 'a' == fromList [(1, 'a')] size (singleton 1 'a') == 1
>>> HashSet.singleton 1 fromList [1]