Macro zkevm_circuits::_matchx

macro_rules! _matchx {
    ($cb:expr, ($($condition:expr => $when:expr),* $(, _ => $catch_all:expr)? $(,)?)) => { ... };
}

matchx Supports _ which works the same as in the normal match: if none of the other arms are active the _ arm will be executed and so can be used to return some default values or could also be marked as unreachable (using the unreachablex! macro).