The intersect function takes the list intersection of two lists. For example,
> [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.