def add( child )
      if child.kind_of? ( XMLDecl )
        if @children[0].kind_of?( XMLDecl )
          @children[0] = child
        else
          @children.unshift child
        end
        child.parent = self
      elsif child.kind_of? DocType
        # Find first Element or DocType node and insert the decl right 
        # before it.  If there is no such node, just insert the child at the
        # end.  If there is a child and it is an DocType, then replace it.
        insert_before_index = @children.find_index { |x|
          x.kind_of?(Element) || x.kind_of?(DocType)
        }
        if insert_before_index # Not null = not end of list
          if @children[ insert_before_index ].kind_of? DocType
            @children[ insert_before_index ] = child
          else
            @children[ insert_before_index-1, 0 ] = child
          end
        else  # Insert at end of list
          @children << child
        end
        child.parent = self
      else
        rv = super
        raise "attempted adding second root element to document" if @elements.size > 1
        rv
      end
    end
def add( child )
      if child.kind_of? ( XMLDecl )
        if @children[0].kind_of?( XMLDecl )
          @children[0] = child
        else
          @children.unshift child
        end
        child.parent = self
      elsif child.kind_of? DocType
        # Find first Element or DocType node and insert the decl right 
        # before it.  If there is no such node, just insert the child at the
        # end.  If there is a child and it is an DocType, then replace it.
        insert_before_index = @children.find_index { |x|
          x.kind_of?(Element) || x.kind_of?(DocType)
        }
        if insert_before_index # Not null = not end of list
          if @children[ insert_before_index ].kind_of? DocType
            @children[ insert_before_index ] = child
          else
            @children[ insert_before_index-1, 0 ] = child
          end
        else  # Insert at end of list
          @children << child
        end
        child.parent = self
      else
        rv = super
        raise "attempted adding second root element to document" if @elements.size > 1
        rv
      end
    end
We override this, because XMLDecls and DocTypes must go at the start
 of the document