Macro gen_many_byref

macro_rules! gen_many_byref {
    (($i:expr, $idx:expr), $l:expr, $f:expr) => { ... };
}

gen_many_byref!(I, Iterable<V>, Fn(I,V)) => I -> Result<I,E> Applies the generator $f to every element of $l, where arguments of $l are references