Macro maybe_backdate 

macro_rules! maybe_backdate {
    (
        ($return_mode:ident, no_backdate, $maybe_default:ident),
        $maybe_update:tt,
        $old_field_place:expr,
        $new_field_place:expr,
        $revision_place:expr,
        $current_revision:expr,
        $zalsa:ident,

    ) => { ... };
    (
        ($return_mode:ident, backdate, $maybe_default:ident),
        $maybe_update:tt,
        $old_field_place:expr,
        $new_field_place:expr,
        $revision_place:expr,
        $current_revision:expr,
        $zalsa:ident,
     ) => { ... };
}

Conditionally update field value and backdate revisions