[tex-eplain] Improvement for hyperlink option handling

[Karl Berry]

Overall it looks nice to me (as usual), but I have a couple questions.

    `type' (type of link/dest) 

What are the types?  href, mailto?  This doesn't really make sense to me.

     `opts' (options for link/dest).

Are the options for things other than controlling appearance?
(Color, boxing, what else?)

    all equation macros which define a
    destination are assigned to the `eq' destination group;

Sounds good.  So what are the other groups?  xref, bib, footnote, url?

    NOTE:  For \hldesttype and \hltype, the `!' is accepted but
           has no effect.  

Beucase the default options for the types is empty?  (In the future it
might not be.)

           The same is true for \hlopts and
           \hldestopts when they are used to set the default
           options.  The reason is that in these cases there
           is nothing to `clear'.

How about having it "set" the default with the ! and "augment" the
default without it?

Usually people do want to keep the defaults and just change what they
want to change.  That's the most reliable approach to keeping compatible
future changes.

    Another four macros will turn links/dests on/off:

      \hldeston
      \hldestoff
      \hlon
      \hloff

Is there a use in controlling links/dests separately?  I can't imagine a
use for creating dests without links, and even less for creating links
without dests.  Would it make sense to reduce to just \hlon+off and have
it control both?

Thanks,
karl