Foreign laws may be proved by parol evidence, but when such law appears to be existing in a written statute or code, it may be rejected unless accompanied by a copy thereof. The unwritten law of any other state or territory of the United States may be proved by parol evidence and by books of reports of cases adjudged in their courts.  

Reference to the citation of such cases shall be deemed to incorporate them in the record. The determination of such law shall be for the court on all the evidence.