Returns denominator of rational number in canonical form.
DESCRIPTION
Given a rational number as supported by the Num library, denominator returns
the denominator $q$ from the rational number cancelled to its reduced form,
$p/q$ where $q > 0$ and $p$ and $q$ have no common factor.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# denominator(Int 22 // Int 7);;
val it : num = 7
# denominator(Int 0);;
val it : num = 1
# denominator(Int 100);;
val it : num = 1
# denominator(Int 4 // Int(-2));;
val it : num = 1