Macro sel_count 

macro_rules! sel_count {
    (
        $arg:ident
    ) => { ... };
    (
        $arg:ident : $($tail:ident :)*
    ) => { ... };
    (
        [$($t:tt)*]
        $arg:ident : $($tail:ident :)*
    ) => { ... };
    (
        [$($t:tt)*]
    ) => { ... };
}