Macro impl_concat 

macro_rules! impl_concat {
    ( @single $target:ident, $n:tt, $m:tt ) => { ... };
    ( $target:ident ; $( ($n:literal, $m:literal) ),* $(,)? ) => { ... };
    ( $target:ident ; $n_start:literal .. $n_end:literal ; $m_start:literal .. $m_end:literal ) => { ... };
}