It may so happen, nonetheless, that the degeneracy is not completely lifted. That is, there exists at least one pair ( a n , b n ) which does not uniquely identify one eigenvector. In this case, we repeat the above process by adding another observable C , which is compatible with both A and B . If the basis of common eigenfunctions of A ^ ^ , B ^ ^ and C ^ ^ is unique, that is, uniquely specified by the set of eigenvalues ( a n , b n , c n ) , then we have formed a CSCO: { A , B , C } . If not, we add one more compatible observable and continue the process till a CSCO is obtained.