In an action for a commission on a sale of land, an offer to show that there was a supplemental contract, and that by an oversight, mistake or fraud, a stipulation that plaintiff's right to sell the land should expire at a time which had passed, was omitted from the writing, contains no offer to prove notice of revocation of plaintiff's right, and a failure to instruct as to what constitutes such notice is not error. Hoffner v. Chambers, 121 Pa. St. 84, 15 A. 492.