Restricting possible connections

[ml <ml knorke in-berlin de>]

Hi,

I think about writing a new dia module, where certain
objects can be connected only to certain others.  (How) can
I disallow connections between arbitrary objects?  I.e. I
have object types A, B, and C - while connections between
A-B and A-C are allowed, but B-C should not be possible.

Many thanks in advance!