((a)) For the purpose of determining the amount of unfunded vested benefits allocable to an employer for a partial or complete withdrawal from a plan which occurs after , and for the purpose of determining whether there has been a partial withdrawal after such date, the amount of contributions, and the number of contribution base units, of such employer properly allocable—
((1)) to work performed under a collective bargaining agreement for which there was a permanent cessation of the obligation to contribute before , or
((2)) to work performed at a facility at which all covered operations permanently ceased before , or for which there was a permanent cessation of the obligation to contribute before that date,
((b)) A plan may, in a manner not inconsistent with regulations, which shall be prescribed by the corporation, adjust the amount of unfunded vested benefits allocable to other employers under a plan maintained by an employer described in subsection (a).