<complexType name="ECParametersType">
  <sequence>
    <element name="FieldID" type="dsig11:FieldIDType" />
    <element name="Curve" type="dsig11:CurveType" />
    <element name="Base" type="dsig11:ECPointType" />
    <element name="Order" type="ds:CryptoBinary" />
    <element name="CoFactor" type="integer" minOccurs="0" />
    <element name="ValidationData"
             type="dsig11:ECValidationDataType" minOccurs="0" />
  </sequence>
</complexType>

...

<complexType name="FieldIDType">
  <choice>
    <element ref="dsig11:Prime" />
    <element ref="dsig11:TnB" />
    <element ref="dsig11:PnB" />
    <element ref="dsig11:GnB" />
    <any namespace="##other" processContents="lax" />
  </choice>
</complexType>