use indexedMap

[iqualfragile]

instead of redundantly saving the index of an element, just use indexedMap, it simplifies the code by removing some deconstruction.
of cause this would remove the uniqueness of the keys.

[evancz]

It's important that the keys are unique. If a change needs to happen to item 4, but item 4 is removed before it is applied, we do not want to accidentally apply it to item 5.