
   I need (probably) to write one or more new Motif widgets on the HP-UX
   platform. Do I need the Motif private header files and source,
   or can I make do with the public headers that are provided?
   "Motif" includes Xt in this context.

Yes.  You'll find it almost impossible without the source
at this point.  It does depend on how ambitious you are and
how concerned you are about compliance with the general
interface and items like traversal.

   One widget is a multi-column list (which lots of people have
   already written, I am sure), and would therefore be probably be
   a subclass of List rather than something simple like an Xt class.
   Is this more difficult (in principle, not lines of code)?

I'm not sure what you're asking.  You could create something
which is very much like a true multi-column list by placing
several lists within a geometry manager,  and putting that
manager within an automatic scrolled window.  This wouldn't
be good for very large lists,  but you might consider this
as an alternative.

   Alternatively, if anyone has a multi-column list widget they
   could sell me, this might save me from having to write one!
   Does it by any chance exist in Motif 1.2 already (I do not
   yet have the spec)?

Motif 1.2 does not have a multi-column list in it.  Have you
looked at commercial sets?  There are also some PD 
widget sets,  one of these might have a multi-column list
you could port.

