/// <summary>Visit a prefix.</summary>
 /// <param name="prefix">Prefix to be visited.</param>
 protected override void VisitPrefix(Prefix prefix)
 {
     _commandText.AppendFormat("PREFIX {0}: <{1}> ", prefix.NamespacePrefix, prefix.NamespaceUri);
 }
 /// <summary>Visit a prefix.</summary>
 /// <param name="prefix">Prefix to be visited.</param>
 protected abstract void VisitPrefix(Prefix prefix);