Andrew Farkas HactarCE

	//! Undo/redo history.
	//!
	//! The simplest model of undo history consists of a single "undo stack." Every
	//! operation pushes a history entry for the previous state onto the undo stack.
	//! The "undo" action pops a history entry from the top of the undo stack and
	//! restores it.
	//!
	//! To add "redo" functionality, we make an additional "redo stack." To undo an
	//! action, we push an entry onto the redo stack and then pop an entry from the
	//! undo stack to restore. To redo an action, we push an entry onto the undo