Tally ** Tally -- Cartesian product of tallies

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