4.6.5 Caution Following that

When using that, we cannot use then after that. It is because we think there may be more than two sentences after such that. When there are more than two sentences, the sentence just before them is not clear.

       assume that A and B:
       then C;

Therefore, the above method of writing is not possible. We have to use labels and write in the following way.

       assume that L1:A and L2:B;
       C by L1,L2;

Or, when only B is used, we can only write as below:

      C by L2;

Also, when there is only one citation, then cannot be used. We have to write by using L2 instead of then. Therefore, when that is stated, labels have to be there or this sentence can not be used. In short, this will not be able to be cited later. Thus, we have to think that the labels will always follow that.
    When that is not used, then can be used. Therefore, the first way of writing,
assume P[x,y] & Q[x,y], in this case, the following can be used.

       assume P[x,y] & Q[x,y];
       then ......

Thus, there is only one sentence without that. For assume, then is used.