Implement a predicate remdup/2 which removes duplicate elements from a list,
indexes the remaining elements (1 to n), and prints out how many elements there are.
For example,
?- remdup([7,2,4,5,6,7,9,4],X).
There are 6 elements in the list.
X = [[1,7],[2,2],[3,4],[4,5],[5,6],[6,9]]
!! the...