Function expected_arm 

fn expected_arm(ty: &Type) -> TokenStream

The from_r::ExpectedArm of one side of an Either, of type ty: the macro’s wording when it has one (crate::r_preconditions::conversion_expectation), a nested Either side by side, NULL or <T> for an Option<T> or Result<T, ()> side, else the type’s __MX_EXPECTATION, with its @param noun (crate::type_inspect::r_value_noun) for when neither the type nor its error names it. Every side is a TryFromSexp type, as Either’s own impl requires (and the Option / Result impls of their T).