21.  Unrealised rent. —

For the purposes of section 21(4), the amount of rent which the owner cannot realise shall be equal to the amount of rent receivable by the assessee but not paid by a tenant of the assessee and so proved to be lost and irrecoverable where, —

a.   the tenancy is bona fide;

b.   the defaulting tenant has vacated, or steps have been taken to compel him to vacate the property;

c.    the defaulting tenant is not in occupation of any other property of the assessee; and

d.    the assessee has taken all reasonable steps to institute legal proceedings for the recovery of the unpaid rent or satisfies the Assessing Officer that legal proceedings would be futile.