Macro gen_from_iterator_impl 

macro_rules! gen_from_iterator_impl {
    (
        vector_type: $type : ident,
        collect_from_type: $collect_from_type : ty,
        underlying_type: $underlying_type : ty,
        r_prefix: $r_prefix : ident,
        SEXP: $sexp : ident
    ) => { ... };
}