note MeowChow You have the right idea. Perhaps this will help you get started: <code> sub rpn_ptree { my@s;$o{$_}?$s[--$#s]=[$_,@s[-2,-1]]:push@s,$_ for@_;@s } </code> It's a pretty well-golfed sub that builds a parse tree out of an RPN expression. The only tricky part is the <tt>--$#s</tt>, which simultaneously pops off the top element of <tt>@s</tt> (by assigning to $#s), and returns the index of the new end of the array. And here's another way to build a parse tree: <code> sub rpn_ptree { my$i;$o{$_[$i]}&&splice@_,$i-=2,3,[@_[$i..$i+2]]while++$i<@_;@_ } </code> <pre> <a href="/index.pl?node=MeowChow"> MeowChow </a> s aamecha.s a..a\u$&owag.print</pre> 81969 82139