-- -*- M2-comint -*- hash: 15511234486960597045

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

i4 : 
