Implement spreadsheet cells with formulas

Design an in-memory spreadsheet that supports these operations on cells such as `A1`, `B2`, and ranges such as `A1:B2`: - `Set(cell, value)`: assign an integer literal to a cell. - `Sum(dest, refs)`: assign `dest` to a formula equal to the sum of all referenced cells. Each item in `refs` is either a single cell like `"B2"` or a rectangular range like `"A1:B2"`. - `Get(cell)`: return the current value of the cell. - `Undo()`: revert the most recent successful write operation. Both `Set` and `Sum` count as write operations. Rules: 1. Cells that have never been assigned should be treated as `0`. 2. Formula cells are reactive: if a referenced cell changes later, `Get` on the formula cell should reflect the new value. 3. Duplicate references count multiple times. 4. A `Sum` operation is invalid if it would introduce a cycle in the dependency graph. In that case, it should raise an error and leave the spreadsheet unchanged. Example: ```text Set("A1", 1) Set("B2", 2) Sum("D1", ["A1:B2", "B2"]) Get("D1") // 5, because A1 + A2 + B1 + B2 + B2 = 1 + 0 + 0 + 2 + 2 Set("A1", 2) Get("D1") // 6 ``` Cycle examples: ```text Sum("A1", ["A1", "B1"]) // invalid: direct self-cycle Sum("A1", ["B1"]) // valid Sum("B1", ["A1"]) // invalid if A1 already depends on B1 Sum("B3", ["B3"]) // invalid ``` Implement the data structure and explain how you would track dependencies, detect cycles, and support `Undo()` efficiently.