[m-rev.] for possible review: tidying up reference manual

[Julien Fischer]

Hi Zoltan,

On Wed, 17 Jul 2019, Zoltan Somogyi wrote:

> The one substantial change is commenting out a paragraph that said that
> solver types couldn't be defined in interface sections. I commented that out
> because the compiler accepts that without complaint.

My immediate reaction to that was: aren't solver types *always* supposed
to be abstract?  I then looked back over the comments on the mailing
lists when the were first introduced (in their current form) and I don't
think the question was addressed.

I haven't checked, but I'm pretty sure that when we used solver types
in G12 all of them were abstract; what's the use case for allowing
them to be defined in the interface?

The other changes look fine BTW.

Julien.