-- -*- M2-comint -*- hash: 3379463902165555509

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 : x-y

o5 = Tally{c => 2}

o5 : Tally

i6 : x'-y'

o6 = VirtualTally{a => -2}
                  c => 2

o6 : VirtualTally

i7 : 
