Update 'Works in progress'

Works-in-progress.md

@@ -17,7 +17,6 @@
         - [ ] `includes`
         - [ ] `equal`
         - [ ] `lexicographical_compare` (using 3-way comparisons)
-        - [ ] `includes`
         - [ ] `begin` and `end`
 - Datastructures
     - [ ] `flat_tree`