code: function(signal, arg) {
              var slot = this._slots[signal];
              if (slot) {
                  slot.call(this, arg, signal);
                  return true;
              }
              return false;
          }
docs: Call the slot mapped to the "signal".
  @param signal : name of the signal on which this object may be registred.
  @param arg : argument to pass to the registred slot.
  @memberof WTag