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

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

i3 : x' = new VirtualTally from x
o3 = VirtualTally{a => 1}
b => 2
c => 3
o3 : VirtualTally

i4 : y' = new VirtualTally from y
o4 = VirtualTally{a => 3}
b => 2
c => 1
o4 : VirtualTally

i5 : xy
o5 = Tally{c => 2}
o5 : Tally

i6 : x'y'
o6 = VirtualTally{a => 2}
c => 2
o6 : VirtualTally
