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What-If Analysis (Data › Forecast): Goal Seek, Scenario Manager, Data Tables; and the Data Tools that work on whole lists: Advanced Filter and Consolidate.
Every what-if evaluation is a trial: the workbook is cloned (cheap — sheets and cell bands are copy-on-write), the input cells are overwritten, and only their dependents are recalculated with the document’s dependency graph. The document itself only changes when a command writes its final answer (one undo step).