forall aa, ab : s . aa <= ab => p
not s(n) <= 0
not s(n) <= 0
s(n) <= 0
i * q <= j * p
i * q <= j * p
c * (a + b) = c * a + c * b
(a + b) * c = a * c + b * c
a <= b <= c
even A x
even A x