One of the arguments may be a Set.
i1 : x = tally {a,a,b} o1 = Tally{a => 2} b => 1 o1 : Tally
i2 : y = tally {1,2,2,2} o2 = Tally{1 => 1} 2 => 3 o2 : Tally
i3 : x ** y o3 = Tally{(a, 1) => 2} (a, 2) => 6 (b, 1) => 1 (b, 2) => 3 o3 : Tally
See also Tally and tally.