function masktree_merge($masktree1, $masktree2, $maskname)
	{
		$merged = array_merge($masktree1, $masktree2);
		if (count($merged) < count($masktree1) + count($masktree2)) {
			$keys1 = array_keys($masktree1);
			$keys2 = array_keys($masktree2);
			$keysm = array_merge($keys1, $keys2);
			$keysc = array_count_values($keysm);
			foreach ($keysc as $keyn => $keyc) {
				if ($keyc > 1) {
					if ($keyn == '0reduce') {
						$this->error(6, 49, 'reduce element already used');
					} elseif ($keyn != '0include') {
						$this->error(16, 60, "template $keyn already defined in <b>$maskname</b>");
					}
				}
			}
		}
		return $merged;
	}