fmod ARE-FIBS is
    protecting INT .
    
    op areFibs : Int Int -> Bool .
    
    vars M N : Int .
    
    eq areFibs(1,1) = true .
    ceq areFibs(M,N) = false if (M < 1) or (N < 1) .
    ceq areFibs(M,N) = false if (M > 1) and (N > 1) and (M >= N) .
    ceq areFibs(M,N) = areFibs(N - M, M) if (M >= 1) and (N > 1) and (N > M) .
endfm