dedup_list

filter out consecutive duplicate elements using the
given relation and return result as a list.

Keep the order of elements unchanged.

ex.
[4,2,2,6,2,1,2,4].dedup_list (a,b -> a%2=b%2) = [4,1,2]
[4,2,2,6,2,1,2,4].dedup_list (<=) = [4,2,1]