Macro impl_commutative 

macro_rules! impl_commutative {
    ($lhs:ty, $trait:ident, $fn_name:ident, $op:tt, $rhs:ty, $($gen:ident: $gen_ty:ident),*) => { ... };
    ($lhs:ty, $trait:ident, $fn_name:ident, $op:tt, $rhs:ty, Output=$out_type:ty, $($gen:ident: $gen_ty:ident),*) => { ... };
}

A macro to implement commutative variants of a binary operation