Macro fix_rs::define_fixt_message

macro_rules! define_fixt_message {
    ( $message_name:ident $( : $message_type:expr => )* { $( $field_required:expr, $field_name:ident : $field_type:ty [$( $version:tt )*] $(=> REQUIRED_WHEN $required_when_expr:expr)* ),* $(),* } ) => { ... };
}