51. An easement extinguished under section 45 revives (a) when the destroyed heritage is, before twenty years have expired, restored by the deposit of alluvion; (b) when the destroyed heritage is a servient building and before twenty years have expired such building is rebuilt upon the same site; and (c) when the destroyed heritage is a dominant building and before twenty years have expired such building is rebuilt upon the same site and in such a manner as not to impose a greater burden on the servient heritage. An easement extinguished under section 46 revives when the grant or bequest by which the unity of ownership was produced is set aside by the decree of a competent Court. A necessary easement extinguished under the same section revives when the unity of ownership ceases from any other cause. A suspended easement revives if the cause of suspension is removed before the right is extinguished under section 47. Illustration A, as the absolute owner of field Y, has a right of way thither over B's field Z. A obtains from B a lease of Z for twenty years. The easement is suspended so long as A remains lessee of Z. But when A assigns the lease to C, or surrenders it to B, the right of way revives.