code: public void addChild(PdfLayer child) {
          if (child.parent != null)
              throw new IllegalArgumentException("The layer '" + ((PdfString)child.get(PdfName.NAME)).toUnicodeString() + "' already has a parent.");
          child.parent = this;
          if (children == null)
              children = new ArrayList();
          children.add(child);
      }
docs: Adds a child layer. Nested layers can only have one parent.
  @param child the child layer