15. Intervention by third parties.- Any person other than the registered proprietor of a trade mark alleging interest in a registered trade mark in respect of which an appeal has been filed under section 91 against the order or decision of the Registrar for the removal of a trade mark or to cancel or vary the registration and to rectify the Register may apply in the prescribed Form 4 under these rules for leave to intervene, stating the nature of his interest and the Deputy Registrar may refuse or grant such leave after hearing, if so required, the parties concerned upon such terms and conditions as he deems fit. | 