Remove-dups
Remove duplicates from a true list and maintain the order of elements.
If l is a true list then (remove-dups l) is a true list with
no duplicates. In contrast with remove-duplicates, the order of
elements is the same. That is, if x and y appear in l
and the first occurrence of x appears before the first occurrence of
y in l, then x appears before y in
(remove-dups l).
