The truth is that there are many, many topics that could be discussed about the leak, but in order not to be tiresome, this will be, for the time being at least, one last question that I would like to address.
It is about invertible counterpoint, or double counterpoint.
What is it? Well, that counterpoint in which the upper voice and the lower voice can be interchanged and still sound good. For this, it is necessary to make sure that the intervals (in strong beats) are consonant both in "normal" and "inverted" position.
The most frequent is to see invertible counterpoint at the octave. That is, two lines that can be inverted by raising one an octave and lowering the other an octave.
But you can also see invertible counterpoint at the twelfth (which is a fifth + octave).
More rarely we will see the tenth (third + octave).
And other intervals are not practical, or very rare.
WHY IS THIS FUNDAMENTAL IN THE FUGUE?
If we compose an invertible counter-subject with the subject, we gain two enormous advantages:
1) In following expositions or entries, we can safely change the order of the subject - countersubject voices.
2) This makes the fugue sound coherent and, for the sake of redundancy, makes it sound like a fugue. Because the fugue tries to reuse the initial material as much as possible.
If anyone is interested, I can post my own appreciations of how to compose these counterpoints. The truth is that mastering the invertible to the octave, you have a long way to go.
To illustrate this, I bring here a fantastic analysis of Bach's C minor fugue where there is not only one invertible counter-subject, but two. Thanks to that, during the development he is placing them up, down, in the middle.....
With this technique you have a great part of the fugue done.