[racket] Detecting cycle in directed graph

From: Erich Rast (erich at snafu.de)
Date: Fri Feb 14 07:42:39 EST 2014

I have a directed graph given as list of pairs ((from-node .
to-node) ...) and need to detect whether adding a pair would create a
cycle.

Is there an easy way to do this already, e.g. some Planet package or
snippet someone would like to share? 

I don't want to reinvent the wheel and this is not a homework question.

Best,

Erich

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