This is mostly a topological sort. If you substitute a7 for a8 it is entirely a topological sort. The question becomes whether cycles exist to refute the ordering.

System tsort will handle this. So will the Graph module. Wolf Book pp304-307 does, too.

After Compline,
Zaxo

In reply to Re: Sorting, given only comparisions by Zaxo
in thread Sorting, given only comparisions by Anonymous Monk

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