code: private PropertySelector buildPropertySelector(GDLParser.PropertyLookupContext ctx) {
      GraphElement element;
  
      String identifier = ctx.Identifier(0).getText();
      String property = ctx.Identifier(1).getText();
  
      if(userVertexCache.containsKey(identifier)) {
        element = userVertexCache.get(identifier);
      }
      else if(userEdgeCache.containsKey(identifier)) {
        element = userEdgeCache.get(identifier);
      }
      else { throw new InvalidReferenceException(identifier);}
  
      return new PropertySelector(element.getVariable(),property);
    }
docs: Builds an property selector expression like alice.age
  
  @param ctx the property lookup context that will be parsed
  @return parsed property selector expression