Spec says:

It is a schema definition error if any children have their dfdl:initiator property set to the empty string.

Does that include a sequence with a hiddenGroupRef?

For example, part of a schema for email headers is this:

                                                                        <xsd:choice dfdl:initiatedContent="yes">

                                                                                    <xsd:element ref="Date"/>

                                                                                    <xsd:element ref="From"/>

                                                                                    <xsd:element ref="MessageId"/>

                                                                                    <xsd:element ref="Subject"/>

                                                                                    <!-- all unknown and unwanted headers -->

                                                                                    <xsd:sequence dfdl:hiddenGroupRef="UnwantedHeadersGroup"/>

                                                                        </xsd:choice>

Date, From, and the other headers are all elements with initiators, but UnwantedHeadersGroup does not have an initiator (because I don’t know it a priori). 

Should this throw an error in a DFDL implementation?  I suspect it should, though I hope it shouldn’t.