i1 : x = tally {a,b,b,c,c,d,d,d}
o1 = Tally{a => 1}
b => 2
c => 2
d => 3
o1 : Tally
|
i2 : x' = new VirtualTally from x
o2 = VirtualTally{a => 1}
b => 2
c => 2
d => 3
o2 : VirtualTally
|
i3 : - x'
o3 = VirtualTally{a => -1}
b => -2
c => -2
d => -3
o3 : VirtualTally
|