Aristotle2 => sub {
		my $str   = shift;
		my $level = shift || return $str;

		$str =~ /.*\((.*?)\)/ and return $1 if $level == -1;

		local $_ = substr $str, 0, -$level;
		my $rx = $rx_hash{$level} ||= do {
			my $rx = join '', '\A', '(?>.*?\()' x $level;
			qr/$rx/;
		};
		m/$rx/g && return substr $_, pos;
	},