A lets a house to B, reserving power to revoke the lease if, in the opinion of a specified surveyor, B…

The illustration to section 42 states that the lease to C "operates as a revocation of B's lease subject to the opinion of the surveyor" on B's use of the house. Under section 42, a later transfer for consideration by a person who reserved a power to revoke works as a revocation of the earlier transfer "subject to any condition attached to the exercise of the power". No separate formal act of revocation is needed, but A cannot ignore the condition that the surveyor must form the opinion.