Adding Proc#change? to check for changes on repeated calling of proc.
Adding Proc#change? to check for changes on repeated calling of proc.

darcs-hash:20071230183818-cbc08-cab1d5073d8c53768b97d1b7489202a936ea5ef7.gz

earthrid/MacBacon,godfat/bacon,chneukirchen/bacon,alloy/MacBacon,jeffkreeftmeijer/bacon