Macro built_in 

macro_rules! built_in {
    (
    $self:ident, $func:ident, $scope:ident, $register_fn:ident;
    $( { $key:literal, $flags:tt, $arity:expr, $name:ident => $block:block $(, $name_a:ident => $block_a:block )? } ),+ $(,)?
  ) => { ... };
}