revised exactly what 4th reviewer mentioned

From the review: 

```
My second problem with the paper is with the writing style. There is the issue
of the informality of some central technical arguments (see also points below
by a second reviewer). But I also had issues with the prose. A technical paper
is a presentation to an audience. When the text reads like a dialog where one
party questions the other, the tone comes across as ravings. Idioms such as
"Unfortunately, we have come across such a counterexample", "Admittedly we
cheat here", "Yes it requires a little more effort", "Now, what is most
interesting is...", etc., are not helping the authors establish credibility. It
took me half the paper to convince myself that the authors know what they are
talking about and they are not just deluded. These are just small instances but
the conversational tone in the writing is rather pervasive.
```

I changed exactly these sentences.