Macro func_internal 

macro_rules! func_internal {
    (
        @internal
        $type:ident
        $([$($cap_ident:ident $(: $cap_expr:expr)?),*])?
        $(|$($param_ident:ident $(: $param_ty:ty)?),*|)?
        $(-> $r_type:ty)?
        $body:block
    ) => { ... };
}