Design a mutable ordered sequence of elements together with a marker that points to one element in the sequence. Support the following operations: - Insert or remove a contiguous block of elements at a given position. - Move a contiguous block of elements from one position to another. - Set the marker to a new position. Explain the API, data model, and how the marker should behave when edits happen before it, after it, or when the marked element is inside a moved or deleted block. Discuss reasonable implementation choices and their time-complexity tradeoffs.