Macro specs_declaration::system

macro_rules! system {
    ($name:ident, |$( $v:ident : $t:ty ),*| $r:block) => { ... };
    ($name:ident <
                   $( $generic:tt $( : $clt:tt $(+ $dlt:tt )* )? ),+
                   >, |$( $v:ident : $t:ty ),*| $r:block) => { ... };
}