New in version 1.1.1.
Return the arithmetic mean of the elements in iterable l.
Where possible return an exact integer.
abjad> l = [1, 2, 2, 20, 30]
abjad> listtools.arithmetic_mean(l)
11
Otherwise return a rational.
abjad> l = [1, 2, 20]
abjad> listtools.arithmetic_mean(l)
Rational(23, 3)
Never return a float.