final ContentAssistContext context, ICompletionProposalAcceptor acceptor) {
String prefix=context.getPrefix();
if(prefix==null || prefix.length()==0 || prefix.charAt(0)!=':'){
return;
}
final ContentAssistContext newContext=context.copy().setMatcher(subStringMatcher).toContext();
//the following seems to be the case if code completion is invoked on an empty prefix
//in this case getting the previous model returns the actual node to be completed!?
if(model instanceof PredicateObjectList){
model=context.getPreviousModel();
}