Is it true that we often use 'not' with a verb or auxiliary verb  i.e. 'not to do', 'do not need to do'? 'not' does not have this limitation when it forms a fixed collocation with other words, such as 'not...but...', 'not only... but also...' 

> 1. The second fact is that many linguists work **not** just on languages
> other than English **but** on languages which have been little studied
> or not studied at all.
> 2. Shakespeare was not only a writer but also an actor.

But I came across this sentence today.

> a. In the discussion of (3) above, we observed that, although we
> analyse clauses and sentences individually for convenience, in real
> language they do not occur in isolation but as part of longer texts.

In my view, it should be 

> b. In the discussion of (3) above, we observed that, although we
> analyse clauses and sentences individually for convenience, in real
> language they occur not in isolation but as part of longer texts.

What's the difference between *a* and *b*? Is *a* formal?