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sub rpn_ptree {
  my@s;$o{$_}?$s[--$#s]=[$_,@s[-2,-1]]:push@s,$_ for@_;@s
}

sub rpn_ptree {
  my$i;$o{$_[$i]}&&splice@_,$i-=2,3,[@_[$i..$i+2]]while++$i<@_;@_
}