public String prettyPrint() {
        final StringBuilder builder = new StringBuilder();
        prettyPrint(builder, "");
        final String pretty = builder.toString();
        return pretty.isEmpty() ? pretty : pretty.substring(0, pretty.length() - 1);
    }
Produce a formatted string representation of the tree structure.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/Tree.java