code: @Nonnull
    public DigestAuthServerBuilder addQOP (@Nonnull final String sQOP)
    {
      if (!HttpStringHelper.isToken (sQOP))
        throw new IllegalArgumentException ("The passed qop-option is not a token: " + sQOP);
  
      m_aQOPs.add (sQOP);
      return this;
    }
docs: This directive is optional, but is made so only for backward compatibility
  with RFC 2069 [6]; it SHOULD be used by all implementations compliant with
  this version of the Digest scheme. If present, it is a quoted string of one
  or more tokens indicating the "quality of protection" values supported by
  the server. The value "auth" indicates authentication; the value "auth-int"
  indicates authentication with integrity protection; see the descriptions
  below for calculating the response directive value for the application of
  this choice. Unrecognized options MUST be ignored.
  
  @param sQOP
  The qop-option to add. May not be <code>null</code>.
  @return this