public String getPredicateDisplayString(SemanticContext.Predicate pred) {
		if ( indexToPredMap==null ) {
			indexToPredMap = getIndexToPredicateMap();
		}
		ActionAST actionAST = indexToPredMap.get(pred.predIndex);
		return actionAST.getText();
	}
Given an arbitrarily complex SemanticContext, walk the "tree" and get display string.
 Pull predicates from grammar text.
tool/src/org/antlr/v4/tool/Grammar.java