Undo/redo is a classic stack pattern that maps naturally to a state machine with history states and a stack context.

• States: canUndo / canRedo / atHead / atTail

• Stack as context with pointer

• Guard: canUndo, canRedo

• Actions: push, undo, redo, clear

• Undo/Redo → Forms — Undoable form edits

• Undo/Redo → Drag and Drop — Undoable reordering

• Case Study: Document Editor — Version history

• Add bounded stack machine (max N entries)

• Add group operations (atomic undo)

• Add visual history timeline