xsl:if

  <xsl:template match="composition">
    <h3><xsl:value-of select="name"/> by <xsl:value-of select="composer"/> </h3>

    <ul>
     <xsl:if test="string(date)">
       <li><xsl:value-of select="date"/></li>
     </xsl:if>
     <xsl:if test="string(length)">
       <li><xsl:value-of select="length"/></li>
     </xsl:if>
     <xsl:if test="string(instruments)">
       <li><xsl:value-of select="instruments"/></li>
     </xsl:if>
     <xsl:if test="string(publisher)">
       <li><xsl:value-of select="publisher"/></li>
     </xsl:if>    
    </ul>

    <p><xsl:value-of select="description"/></p>    
    
  </xsl:template>

• The string() function converts the value of the element to a string

• The number() function converts the value of the element to a number

• Zero length strings are false

• There are all the <, >, =, !=, <= and >= operators you expect