29 U.S.C. § 1397

1. (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 September 25, 1980, 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—shall not be taken into account.
   1. (1)to work performed under a collective bargaining agreement for which there was a permanent cessation of the obligation to contribute before September 26, 1980, or
   2. (2)to work performed at a facility at which all covered operations permanently ceased before September 26, 1980, or for which there was a permanent cessation of the obligation to contribute before that date,
2. (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).