private function exportDotRenderGroups($groups, $group_content, $indent = "\t") {
		foreach ($groups as $g => $group) {
			echo $indent, "subgraph ", $this->exportDotIdentifier($g, 'cluster_'), " {\n";
			if (isset($group['label'])) {
				echo $indent, "\t", "label = \"", addcslashes($group['label'], '"'), "\";\n";
			}
			if (!empty($group['color'])) {
				echo $indent, "\t", "color=\"", addcslashes($group['color'], '"'), "\";\n";
				echo $indent, "\t", "fontcolor=\"", addcslashes($group['color'], '"'), "\";\n";
			} else {
				// This cannot be defined globally, since nested groups inherit the settings.
				echo $indent, "\t", "color=\"#666666\";\n";
				echo $indent, "\t", "fontcolor=\"#666666\";\n";
			}
			foreach ($group_content[$g] as $s) {
				echo $indent, "\t", $this->exportDotIdentifier($s), ";\n";
			}
			if (isset($group['groups'])) {
				$this->exportDotRenderGroups($group['groups'], $group_content, "\t".$indent);
			}
			echo $indent, "}\n";
		}
	}