docs: Expression XreadDateTimeValueFunctionOrNull() {
  
          FunctionSQL function = null;
  
          switch (token.tokenType) {
  
              case Tokens.CURRENT_DATE :
              case Tokens.CURRENT_TIME :
              case Tokens.CURRENT_TIMESTAMP :
              case Tokens.LOCALTIME :
              case Tokens.LOCALTIMESTAMP :
                  function = FunctionSQL.newSQLFunction(token.tokenString,
                                                        compileContext);
                  break;
  
              case Tokens.NOW :
              case Tokens.TODAY :
                  function = FunctionCustom.newCustomFunction(token.tokenString,
                          token.tokenType);
                  break;
  
              default :
                  return null;
          }
  
          if (function == null) {
              throw super.unexpectedToken();
          }
  
          return readSQLFunction(function);
      }