ISBN2 by Mark Byers

#!ruby -p
$_[n= ~/\?/]=?0
$_[n]+=1while(eval'+$_[4+_-=1]%??*~[_,_*2&3][l&1]'*_=l= ~/$/-4)%(9^l/4)>_

Note that non-ascii characters in the above source code will be escaped (such as \x9f).

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return top