Can an agency be deemed irrevocable unilaterally by the donor?

Counsel for the applicant does not dispute the general principle that where an agency is coupled with an interest it is irrevocable unilaterally by the donor. Counsel, however, in the present case argues that the agency is not coupled with an interest in that it only arises upon default of the applicant or donor and is therefore an authority that has been given independently of the interest and of the type referred to in the final lines of the reference herein from the judgment in Smart v. Sandars, supra.