Christiaan,
Just adding a bit to what Dennis said about references. In your case it looks like you need to do this checking twice, since you have A depending on B depending back on A. Yes, it is a bad idea to have circular references, in particular, and any external references that aren't necessary or desired. So called top-down design techniques can take great advantage of external references, in a large variety of ways, but you really need to know what you are doing and what the resulting dependencies will mean for model independence, document management systems, etc.
David
