// ------------------------------ SignXps -----------------------------
/// <summary>
///   Signs an XPS document with a given X509 certificate, and if one
///   exists, associates the signature with a given SignatureDefintion
///   spotId GUID.</summary>
/// <param name="xpsDocument">
///   The XPS document to sign.</param>
/// <param name="cert">
///   The X509 certificate to use for signing.</param>
/// <param name="spotId">
///   The nullable spotId GUID of the signature definition.</param>
public void SignXps(
    XpsDocument xpsDocument, X509Certificate cert, Guid? spotId )
{
    // If there's a SignatureDefinition spotId,
    // associate it with the signature.
    if (spotId != null)
    {
        xpsDocument.SignDigitally(
            cert, true, XpsDigSigPartAlteringRestrictions.None,
            spotId.Value);
    }
    else
    {
        xpsDocument.SignDigitally(
            cert, true, XpsDigSigPartAlteringRestrictions.None);
    }
}// end:SignXps()