private void doIndent() throws XMLStreamException {
            if (depth > 0) {
                for (int i = 0; i < depth; i++)
                    super.writeCharacters(indentStep);
            }
        }
Print indentation for the current level.

@exception org.xml.sax.SAXException
             If there is an error writing the indentation characters, or if
             a filter further down the chain raises an exception.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphml/GraphMLWriterHelper.java