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/// The signal sent when the record, the history, or the receiver changes.
///
/// When one of these states changes, they will send a corresponding signal to the user.
/// For example, if the record can no longer redo any commands, it sends a `Redo(false)`
/// signal to tell the user.
#[derive(Copy, Clone, Debug, Hash, Ord, PartialOrd, Eq, PartialEq)]
pub enum Signal {
    /// Says if the record can undo.
    ///
    /// This signal will be emitted when the records ability to undo changes.
    Undo(bool),
    /// Says if the record can redo.
    ///
    /// This signal will be emitted when the records ability to redo changes.
    Redo(bool),
    /// Says if the receiver is in a saved state.
    ///
    /// This signal will be emitted when the record enters or leaves its receivers saved state.
    Saved(bool),
    /// Says if the current command has changed.
    ///
    /// This signal will be emitted when the cursor has changed. This includes
    /// when two commands have been merged, in which case `old == new`.
    Cursor {
        /// The old cursor.
        old: usize,
        /// The new cursor.
        new: usize,
    },
    /// Says if the current branch has changed.
    ///
    /// This is only emitted from `History`.
    Branch {
        /// The old branch.
        old: usize,
        /// The new branch.
        new: usize,
    },
}