(810 ILCS 5/2A-202) (from Ch. 26, par. 2A-202)
    Sec. 2A-202. Final written expression; parol or extrinsic evidence. Terms with respect to which the confirmatory memoranda of the parties agree or which are otherwise set forth in a writing intended by the parties as a final expression of their agreement with respect to such terms as are included therein may not be contradicted by evidence of any prior agreement or of a contemporaneous oral agreement but may be explained or supplemented:
        (a) by course of dealing or usage of trade or by
course of performance; and
        (b) by evidence of consistent additional terms unless
the court finds the writing to have been intended also as a complete and exclusive statement of the terms of the agreement.
(Source: P.A. 87-493.)