/**
  * Check whether a lesson is part of a course
  *
  * @param mixed lesson A lesson id or an EfrontLesson object
  * @since 3.6.3
  * @access public
  */
 public function isCourseLesson($lesson)
 {
     $lesson = EfrontLesson::convertArgumentToLessonObject($lesson);
     $result = eF_getTableData("lessons_to_courses", "*", "lessons_ID=" . $lesson->lesson['id'] . " and courses_ID=" . $this->course['id']);
     return !empty($result);
 }