@Override
    public SummaryField clone() {
        try {
            SummaryField clone = (SummaryField)super.clone();
            if (this.sources != null)
                clone.sources = new LinkedHashSet<>(this.sources);
            if (this.destinations != null)
                clone.destinations = new LinkedHashSet<>(destinations);
            clone.unresolvedType = unresolvedType;
            return clone;
        }
        catch (CloneNotSupportedException e) {
            throw new RuntimeException("Programming error");
        }
    }
Returns a string which aids locating this field in the source schema
config-model/src/main/java/com/yahoo/vespa/documentmodel/SummaryField.java