-- > [1,2,3,4] `intersect` [2,4,6,8] == [2,4]
--
-- If the first list contains duplicates, so will the result.
+--
+-- > [1,2,2,3,4] `intersect` [6,4,4,2] == [2,2,4]
+--
-- It is a special case of 'intersectBy', which allows the programmer to
-- supply their own equality test.