3.4 New Item: unify simple and extended links

During the meeting of March 1st, the ERB agreed to package up xlink
locators in subelements.  Jean Paoli pointed out [but I have agreed 
to bring this forward since he's on the road] that the declarations
and attlists for simple and extended links are VERY similar; and it
might be appealing to allow one locator to exist within the
start-tag of an extended link, such that an extended link is just
a simple link with child elements.  E.g.

<a role="3-way" href="#lab1"><extra href="#lab2"><extra href="#lab3"></a>

One virtue of this is that we can go to the web-heads and say
"not only have we given you a powerful extended link facility, but
you can do it just by adding children to your existing <A elements".

Some problems come up:

- is the locator that's in the parent element somehow distinguished
  or special, as opposed to all those in the child elements?
- since a link containing another link <A href="foo"><IMG SRC="BAR"></A>
  is different from an extended link, we'd still need the archform-like
  attribute to tell them apart
- it would be appealing, in an xlink with locators in child elements, to
  use ROLE or HRTYPE attributes in the parent linking element to
  establish defaults for all the child elements... does having a
  locator in the parent screw this up?

Input requested.