nil assignment

proto infix:<=>($, $) is rw { * }
multi infix:<=>(Mu \a, Mu \b) is rw {
    nqp::p6store(a, b)
}
multi infix:<=>(Mu \a, Nil \b) is rw {
    if nqp::iscont(a) {
        nqp::p6store(a, a.VAR.of);
    }
    else { X::Assignment::RO.new.throw; }
}