Stencil modification or Trim Function
 

I am trying to create some objects and this should be easily done using two stencils with a trim function. However I cannot see a Trim Function in Omnigraffle. For example, if I want a rectangle with an semi-circular end, then how can I use the rectangle and circle stencils, set their sizes, bring them together and then trim off the appropriate end of the rectangle and half of the circle, then I would group as the desired object? Comments appreciated.

In the OG (Professional version only) menu system:

Edit > Shapes > [Subtract | Union | Intersect]

Thank you very much. I'm up and running again! Easy when you know how.