Fix a silly bug, recently introduced, which prevented prox modifiers

Fix a silly bug, recently introduced, which prevented prox modifiers
from being properly emitted as XCQL.  I'd wrongly added the new argument,
Vector prefixes, to CQLProxNode.opXCQL(), so that its type signature
was different from that of CQLBooleanNode.opXCQL() and it didn't
override the base class's functionality.

Also, fix a minor typo: renamed opXQL() to opCXQL(), which is what I
intended all along.  Just a slip of the finger.