Whenever a claim or defense is founded upon a written instrument other than a policy of insurance, a copy of such instrument or the pertinent parts thereof shall be attached to the pleading as an exhibit unless the instrument is (1) a matter of public record in the county in which the action is commenced and its location in the record is set forth in the pleading; (2) in the possession of the adverse party and this fact is stated in the pleading; (3) inaccessible to the pleader or is of such nature that attaching the instrument would be unnecessary or impracticable and this fact is stated in the pleading, together with the reason therefor. Every exhibit so attached or referred to under (1) and (2) shall be a part of the pleading for all purposes.