private List<String> toStringList(Iterator<SummaryField.Source> i) {
        List<String> ret = new ArrayList<>();
        while (i.hasNext()) {
            ret.add(i.next().getName());
        }
        return ret;
    }
Don't include field in map if sources are the same as the struct sub fields for the SDField.
But do map if not all do summarying.
Don't map if not struct either.
@param summaryField a {@link SummaryField}
config-model/src/main/java/com/yahoo/schema/derived/VsmSummary.java