FastChords has one level of undo.  Meaning when undo is active only the last operation performed can be reversed.

Here are some points about undo:

Undo is activated when operations affect the contents of the chords table.

When Undo is active, the undo button is clear, when undo is not active, the button is "fuzzy".

When undo is active and the undo operation is performed, the undo button becomes "fuzzy", meaning undo becomes inactive until the next program action that changes the chord table.

when undo is performed, the entire contents of the chords table is restored to its previous state.