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