QGraph prohibits constraints between two annotated query elements. As we saw in Chapter 4, Numeric Annotations, annotating adjacent vertices can result in ambiguities of interpretation. Placing a constraint on two annotated elements results in similar ambiguities.

QGraph admits one exception to the above prohibition against constraints between two annotated elements: Constraints between an annotated vertex and its adjacent annotated edge are permitted. This exception has not been implemented in Proximity, however. Proximity prohibits all constraints between two annotated elements, whether or not they involve this special case.

Proximity imposes additional restrictions on the use of constraints for annotated elements. See “Implementation in Proximity” later in this chapter for details on these limitations.