code: void addRightChild(final byte b, @NotNull final MutableNode child) {
          final ChildReference right = children.getRight();
          if (right != null && (right.firstByte & 0xff) >= (b & 0xff)) {
              throw new IllegalArgumentException();
          }
          children.putRight(new ChildReferenceMutable(b, child));
      }
docs: Adds child which is greater (i.e. its next byte greater) than any other.
  
  @param b     next byte of child suffix.
  @param child child node.