Add CAV accept.

bibs/header.bib

@@ -644,6 +644,7 @@
 @STRING{cav15       = cav # ", San Francisco, California" }
 @STRING{cav16       = cav # ", Toronto, Ontario" }
 @STRING{cav17       = cav # ", Heidelberg, Germany" }
+@STRING{cav18       = cav # ", Oxford, England" }
 % ---
 @STRING{cp          = "International Conference on Principles and Practice of
                        Constraint Programming (CP)" }