code: protected <IPW extends IndentingPrintWriter> IPW writeChildrenTo(IPW output) {
          Collection<? extends UMLNode> children = getChildren();
          if (!children.isEmpty()) {
              IndentingPrintWriter indented = output.indent();
              children.forEach(child -> child.writeTo(indented));
          }
          return output;
      }
docs: Helper method to write all children to the specified output.
  <p>
  By default children will be {@link #writeTo(IndentingPrintWriter) written}
  with increased indentation for legibility.
  
  @param <IPW>  The subclass of indenting print writer being written to.
  @param output The output to write the children to.
  @return A reference to the output for method chaining purposes.