ChainComplex ++ ChainComplex -- direct sum

i1 : C = resolution cokernel matrix {{4,5}}

       1       1
o1 = ZZ  <-- ZZ
              
     0       1

o1 : ChainComplex

i2 : C ++ C[-2]

       1       1       1       1
o2 = ZZ  <-- ZZ  <-- ZZ  <-- ZZ
                              
     0       1       2       3

o2 : ChainComplex