815. Contingent liabilities.

In computing a gain, no allowance is made in the first instance in respect of a contingent liability of the following three kinds:


    (1)     any liability remaining with, or assumed by, a person assigning a lease of land or other property which is contingent on a default in respect of liabilities assumed by the assignee under the terms and conditions of the lease1;


    (2)     any contingent liability in respect of any covenant for quiet enjoyment or other obligation assumed by a person as vendor or lessor of land, or of any estate or interest in land2;


    (3)     any contingent