Function guarded 

fn guarded(param: &str, ty: &Type, out: Vec<RAssertion>) -> Vec<RAssertion>

Let an Option<T> parameter pass NULL, and a Missing<T> parameter an omitted argument, in the named checks out on parameter param of type ty: each condition gains an is.null(param) || / missing(param) || prefix. The failure message names only the failed requirement: it is shown only for a present, non-NULL value.