Tally + Tally -- union of tallies

i1 : x = tally {a,a,a,b,b,c}

o1 = Tally{a => 3}
           b => 2
           c => 1

o1 : Tally

i2 : y = tally {b,c,c,d,d,d}

o2 = Tally{b => 1}
           c => 2
           d => 3

o2 : Tally

i3 : x + y

o3 = Tally{a => 3}
           b => 3
           c => 3
           d => 3

o3 : Tally