Tally + Tally

x + y -- produces union of two tallies.

One of the arguments may be a Set.

     i1 = x = tally {a,a,a,b,b,c}
     
     o1 = tally {3 : a, 2 : b, 1 : c}
     
     o1 : Tally

     i2 = y = tally {b,c,c,d,d,d}
     
     o2 = tally {1 : b, 2 : c, 3 : d}
     
     o2 : Tally

     i3 = x + y
     
     o3 = tally {3 : a, 3 : b, 3 : c, 3 : d}
     
     o3 : Tally