Except as provided in subsection (b) of this section evidence is admissible in any proceeding to prove the execution of a written instrument attested by one or more subscribing witnesses in the same manner as the instrument might be proved had it not been attested. Evidence of a disputed writing is admissible and may be submitted to the trier of the facts for its determination as to genuineness. The provisions of this section do not apply to the proof of the execution of a last will and testament or codicil.