code: public boolean isExecutable(String activity) throws CompatibilityException {
                  validateActivity(activity);
                  return acModel != null && acModel.isExecutable(activity);
          }
docs: Checks if the given activity is executable, i.e. there is at least
  one subject which is authorized to execute it.<br>
  This method delegates the call to the access control model.
  
  @param activity The activity in question.
  @return <code>true</code> if the given activity is executable;<br>
  <code>false</code> otherwise.
  @throws ParameterException
  @throws CompatibilityException