public String joinPredicateOperands(SemanticContext.Operator op, String separator) {
		StringBuilder buf = new StringBuilder();
		for (SemanticContext operand : op.getOperands()) {
			if (buf.length() > 0) {
				buf.append(separator);
			}

			buf.append(getSemanticContextDisplayString(operand));
		}

		return buf.toString();
	}
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