-----Description-----
Given a sorted list of distinct integers and a target value, return the index if the target is found. If it is not found, return the index where it would be inserted to maintain the sorted order.

This function must preserve the sorted property of the list. The list is assumed to be strictly increasing and contain no duplicates.

-----Input-----
xs : List Int — a sorted list of distinct integers in increasing order
target : Int — the integer to search for

-----Output-----
A natural number (Nat) representing the index at which the target is found, or the index at which it should be inserted to maintain sorted order.

