public boolean hasExplicitSingleSource() {
        String fieldName = getName();
        String sourceName = getSingleSource();
        if (fieldName.equals(sourceName)) {
            return false;
        }
        if (sourceName.contains(".")) {
            return false;
        }
        if (sources.size() > 1) {
            return false;
        }
        return true;
    }
Returns true if the summary field uses an explicit source, i.e.
a field with different name that is not a nested field.
config-model/src/main/java/com/yahoo/vespa/documentmodel/SummaryField.java