Macro idempotency_guard

macro_rules! idempotency_guard {
    ($events:expr, $( $pattern:pat $(if $guard:expr)? ),+ $(,)?) => { ... };
    ($events:expr, $( $pattern:pat $(if $guard:expr)? ),+,
     => $break_pattern:pat $(if $break_guard:expr)?) => { ... };
}