Write a Python function has_subsequence(a,s) that determines if the list (or string), a, contains s as a (not necessarily consecutive) subsequence.

Examples

• 'here we go again' contains 'we gain' as a subsequence.
• 'green eggs and ham sandwhiches' contains 'reggae' as a subsequence.

Your function should return an array of indices that shows the location of the elements of s in a. In the 'here we go again' example this array would contains [5,6,12,13,14,15].

If you can't find s as a subsequence of a, then you should return None.