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29 U.S. Code § 1392 - Obligation to contribute

(a) “Obligation to contribute” definedFor purposes of this part, the term “obligation to contribute” means an obligation to contribute arising—
under one or more collective bargaining (or related) agreements, or
as a result of a duty under applicable labor-management relations law, but
does not include an obligation to pay withdrawal liability under this section or to pay delinquent contributions.
(b) Payments of withdrawal liability not considered contributions

Payments of withdrawal liability under this part shall not be considered contributions for purposes of this part.

(c) Transactions to evade or avoid liability

If a principal purpose of any transaction is to evade or avoid liability under this part, this part shall be applied (and liability shall be determined and collected) without regard to such transaction.