docs: private AbstractExpression parseExpressionNode(VoltXMLElement exprNode) {
          String elementName = exprNode.name.toLowerCase();
  
          XMLElementExpressionParser parser = m_exprParsers.get(elementName);
          if (parser == null) {
              throw new PlanningErrorException("Unsupported expression node '" + elementName + "'");
          }
          AbstractExpression retval = parser.parse(this, exprNode);
          assert("asterisk".equals(elementName) || retval != null);
          return retval;
      }