code: private ComparableExpression extractComparableExpression(GDLParser.ComparisonElementContext element) {
      if(element.literal() != null) {
        return new Literal(getPropertyValue(element.literal()));
      } else if(element.propertyLookup() != null) {
        return buildPropertySelector(element.propertyLookup());
      } else {
        return new ElementSelector(element.Identifier().getText());
      }
    }
docs: Extracts a ComparableExpression from comparissonElement
  
  @param element comparissonElement
  @return extracted comparable expression