Solution

sits_right_of(ron,natalie).
sits_right_of(hermione,ron).
sits_right_of(harry,hermione).
sits_right_of(colin,harry).
sits_right_of(seamus,colin).
sits_right_of(angelina,seamus).
sits_right_of(ginny,angelina).
sits_right_of(dean,ginny).
sits_right_of(dennis,dean).
sits_right_of(lee,dennis).
sits_right_of(george,lee).
sits_right_of(fred,george).
sits_right_of(alicia,fred).
sits_right_of(neville,alicia).
sits_right_of(lavender,neville).
sits_right_of(parvati,lavender).
sits_right_of(katie,parvati).
sits_right_of(natalie,katie).
    

Back to the exercise.