[m-rev.] for post-commit review: improve digraph.m

[Peter Wang]

On Thu, 20 Jan 2022 20:58:26 +1100 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> For review by anyone.
> 
> Zoltan.

> Improve the diggraph module.
> 
> Fix a mistake in the comment on an exported predicate.
> Improve the wording in other comments.
> 
> Use explicit module qualification in bunch of places, because without them
> references to predicate names that occur in many standard library modules
> (that digraph.h imports) are ambiguous to human readers, though not to
> the compiler.
> 
> Replace some functions with state-variable-friendly predicates.

That's fine.

Peter