[Xy-pic] binary trees

[Andreas Bauer]

Hello,

I hope someone on this list can help me with drawing a bunch of binary trees. 
In fact, the ordinary tree is not a problem, but I'd like to use these 
"roof-like" triangle structures that represent entire subtrees. (Use case: 
B-Trees, AVL-Trees, etc.) I just don't know how to draw them.

Consider this example: If I have a root node A and a right node B, then I 
want my left edge attached to a triangle structure, representing a whole 
subtree. How do I attach the graphics for this subtree to root node A?

         A
        / \
       /   \
      / \   B
      ---
      sub.

I asked this on a newsgroup before, with no success. Also I Googled around, 
but with no luck either...

I'd appreciate your hints and help. Thank you very much!
Andreas Bauer.