Zero or One Children ?

  <composition composer="c1">
    <title>Trio for Flute, Viola and Harp</title>
    <date><year>1994</year></date> 
    <length>13'38"</length>
    <instruments>fl, hp, vla</instruments>
    <description>
      <p>Premiered at Queens College in April, 1996 by Sue Ann Kahn, 
      Christine Ims, and Susan Jolles. In 3 movements :</p>
      <ul>
        <li>mvt. 1: 5:01</li>
        <li>mvt. 2: 4:11</li>
        <li>mvt. 3: 4:26</li>
      </ul>  
    </description>
    <publisher>Theodore Presser</publisher>
  </composition>

• Suffixing an element name with a question mark (?) in the content model indicates that either 0 or 1 (but not more than one) of that element are expected at that position

<!ELEMENT composition 
   (title, date, length?, instruments, description?, publisher?)>