The subscribing witness to prove execution of private documents must be produced in all cases except the following, unless otherwise specially provided:

1. (1) Ancient writings which prove themselves;
2. (2) If from any cause the witness cannot be produced or sworn;
3. (3) Official bonds required by law to be approved or tested by a particular functionary;
4. (4) If the paper is only incidentally or collaterally material to the case;
5. (5) If the party executing the written instrument testifies to its execution; or
6. (6) If the document is self-proving or properly acknowledged.

(Code 1907, §4004; Code 1923, §7702; Code 1940, T. 7, §416.)