predicates
      % Fields: Fill name, academic title, courses
      nondeterm professors(symbol, symbol, symbol).
      % Fields: name of lesson, professor's name, time
      nondeterm lessons(symbol, symbol, symbol).
      % Fields: name of professor1, name of professor2
      nondeterm getChange(symbol, symbol, symbol).
      % Fields: professor name, lesson time
      nondeterm getlesson(symbol, symbol, symbol).
      nondeterm getProfessor(symbol, symbol).
clauses
     % Get lesson name by time and professor name
     getLesson(Name, Time, LessonName):- lessons(LessonName, Name, Time).
     % Get professor name by lesson name
     getProfessor(LessonName, ProfessorName):- professors(ProfessorName, _, LessonName).
     % Get professor name, who can change another professor in Time
     getChange(Name, Time, Name2):- getLesson(Name, Time, LessonName), getProfessor(LessonName, Name2), Name<>Name2, lessons(LessonName, Name2, Time2), Time<>Time2.
     professors("professor_1", "professor", "lesson_6").
     professors("professor_2", "professor", "lesson_6").
     lessons("lesson_6", "professor_1", "00:00").
     lessons("lesson_6", "professor_2", "01:00").
     lessons("lesson_6", "professor_2", "02:00").
GOAL
     % getLesson("professor_1","00:00", LessonName).
     % getProfessor("lesson_6", Professor).
     
     getChange("professor_1", "00:00", Name2).