========================================================================
Omit Node
========================================================================

/ {
    /omit-if-no-ref/ child {};
};

---

(document
    (node
        name: (identifier)
        (omit_if_no_ref
            (node name: (identifier))
        )
    )
)

========================================================================
Omit Labeled Node
========================================================================

/ {
    /omit-if-no-ref/ label: child {};
};

---

(document
    (node
        name: (identifier)
        (omit_if_no_ref
            (node
                label: (identifier)
                name: (identifier)
            )
        )
    )
)

========================================================================
Omit Reference
========================================================================

/omit-if-no-ref/ &node;

---

(document
    (omit_if_no_ref
        (reference
            label: (identifier)
        )
    )
)