Notwithstanding any contract to the contrary, whereany firm is liable to pay any

.. ?.a. -... tax, interest or penalty under this Regulation, the firm and each of the partners of the firm shall be jointly and severally liable for such payment: .. , : "* .? . , Provided that where any such partner retires from the firm, he shall intimate the date

of his retirement to the Commissioner by a notice to that effect in writing and he shall be

liable to pay tax, interest or penalty remaining unpaid at the time of his retirement and any tax, interest or penalty due up to the date of his retirement though unassessed on that date: ~rovidedfurther that ifno such intimation is given within fifteen days from the date of retirement, the liability of the partner under the first proviso shall continue until the date on which such intimation is received by the Commissioner.