Macro flex_error::define_error_constructor

macro_rules! define_error_constructor {
    ( $tracer:ty;
    $name:ident;
    $suberror:ident;
    ( $( $arg_name:ident: $arg_type:ty ),* )
  ) => { ... };
    ( $tracer:ty;
    $name:ident;
    $suberror:ident;
    ( $( $arg_name:ident: $arg_type:ty ),* )
    [ $source:ty ]
  ) => { ... };
}