>I'm not sure if you're aware  that you're asking for 
>what's known as a "topological sort" ...
> efficient solution has linear complexity: O(m+n) 

Precisely. For my purposes, I would go so far as to suggest
<hint>some of these graph algorithms should be available as 
some type of extension function</hint>. Here is a fairly 
comprehensive list of algorithms one might draw from:

   http://hissa.nist.gov/dads/

Regards,

Dan

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