(a) Any pleading, although not required to be sworn to, may be verified by the oath of the party filing it or of any other person or persons having knowledge of the facts pleaded. Corporations may verify by the oath of any officer or agent having knowledge of the facts. If any pleading is so verified, every subsequent pleading must also be verified, unless verification is excused by the court. In pleadings which are so verified, the several matters stated shall be stated positively or upon information and belief only, according to the fact. Verified allegations do not constitute evidence except by way of admission.
     (b) The allegation of the execution or assignment of any written instrument is admitted unless denied in a pleading verified by oath, except in cases in which verification is excused by the court. If the party making the denial is not the person alleged to have executed or assigned the instrument, the denial may be made on the information and belief of that party.