This is a "pure Ruby" implementation of the FXUndoList and FXCommand classes from the standard FOX distribution. Since those classes are independent of the rest of FOX this is a simpler (and probably more efficient) approach than trying to wrap the original C++ classes.

Notes (by Jeroen, lifted from FXUndoList.cpp):

• When a command is undone, it's moved to the redo list.
• When a command is redone, it's moved back to the undo list.
• Whenever adding a new command, the redo list is deleted.
• At any time, you can trim down the undo list down to a given maximum size or a given number of undo records. This should keep the memory overhead within sensible bounds.
• To keep track of when we get back to an "unmodified" state, a mark can be set. The mark is basically a counter which is incremented with every undo record added, and decremented when undoing a command. When we get back to 0, we are back to the unmodified state.

  If, after setting the mark, we have called FXUndoList#undo, then the mark can be reached by calling FXUndoList#redo.

  If the marked position is in the redo-list, then adding a new undo record will cause the redo-list to be deleted, and the marked position will become unreachable.

  The marked state may also become unreachable when the undo list is trimmed.

• You can call also kill the redo list without adding a new command to the undo list, although this may cause the marked position to become unreachable.
• We measure the size of the undo-records in the undo-list; when the records are moved to the redo-list, they usually contain different information!