How to add points (corners) to a polygon?

[Keith Bremer <keith bremer ntlworld com>]

Dear Dia People,

   This program is a really nice little tool - I use it quite a lot to 
produce UML, ERD and database design models.  However, I've encountered 
a situation where I'd really like to draw a >3-sided polygon and I can't 
see how.

   When I add a polygon to a diagram I get a triangle - fine as far as 
it goes, but I was expecting to be able to add new corner points to the 
object.  How do I do that?  I've tried almost every key combination I 
can think of while clicking the mouse on the edge I want to extend, but 
without any effect.  I'm sure It's just a simple thing but i can't 
figure it out - the FAQ doesn't seem to mention polygons, and the 
documentation doesn't say how as far as I can see.

   Any help will be much appreciated.

      Thanks in advance,
      Keith Bremer.