@Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        if (!super.equals(o)) {
            return false;
        }
        ReferenceFieldValue that = (ReferenceFieldValue) o;
        return Objects.equals(referenceType, that.referenceType) &&
                Objects.equals(documentId, that.documentId);
    }
Creates a reference pointing to a particular document instance in the document
type referenced by <code>referenceType</code>.

@param referenceType reference target type
@param documentId document ID of the same document type as that given by <code>referenceType</code>
@throws IllegalArgumentException if documentId is not of the expected document type
document/src/main/java/com/yahoo/document/datatypes/ReferenceFieldValue.java