def write_c_nv_pr(id, name, descr = nil)
      attributes = [
          ['id',   id],
          ['name', name]
      ]

      # Add description attribute for images.
      attributes << ['descr', descr] if descr

      @writer.empty_tag('xdr:cNvPr', attributes)
    end
def write_c_nv_pr(id, name, descr = nil)
      attributes = [
          ['id',   id],
          ['name', name]
      ]

      # Add description attribute for images.
      attributes << ['descr', descr] if descr

      @writer.empty_tag('xdr:cNvPr', attributes)
    end
Write the <xdr:cNvPr> element.