((a)) ** Single-employer plans with two or more contributing sponsors** Except as provided in subsection (d), the plan administrator of a single-employer plan which has two or more contributing sponsors at least two of whom are not under common control—
((1)) shall notify the corporation of the withdrawal during a plan year of a substantial employer for such plan year from the plan, within 60 days after such withdrawal, and
((2)) request that the corporation determine the liability of all persons with respect to the withdrawal of the substantial employer.
((b)) ** Computation of liability** Except as provided in subsection (c), any one or more contributing sponsors who withdraw, during a plan year for which they constitute a substantial employer, from a single-employer plan which has two or more contributing sponsors at least two of whom are not under common control, shall, upon notification of such contributing sponsors by the corporation as provided by subsection (a), be liable, together with the members of their controlled groups, to the corporation in accordance with the provisions of and this section. The amount of liability shall be computed on the basis of an amount determined by the corporation to be the amount described in for the entire plan, as if the plan had been terminated by the corporation on the date of the withdrawal referred to in subsection (a)(1) multiplied by a fraction—
((1)) the numerator of which is the total amount required to be contributed to the plan by such contributing sponsors for the last 5 years ending prior to the withdrawal, and
((2)) the denominator of which is the total amount required to be contributed to the plan by all contributing sponsors for such last 5 years.
((c)) ** Bond in lieu of payment of liability; 5-year termination period**
((1)) In lieu of payment of a contributing sponsor’s liability under this section, the contributing sponsor may be required to furnish a bond to the corporation in an amount not exceeding 150 percent of his liability to insure payment of his liability under this section. The bond shall have as surety thereon a corporate surety company which is an acceptable surety on Federal bonds under authority granted by the Secretary of the Treasury under sections 9304–9308 of title 31. Any such bond shall be in a form or of a type approved by the Secretary including individual bonds or schedule or blanket forms of bonds which cover a group or class.
((2)) If the plan is not terminated under section 1341(c) or 1342 of this title within the 5-year period commencing on the day of withdrawal, the liability is abated and any payment held in escrow shall be refunded without interest (or the bond cancelled) in accordance with bylaws or rules prescribed by the corporation.
((3)) If the plan terminates under section 1341(c) or 1342 of this title within the 5-year period commencing on the day of withdrawal, the corporation shall—
((A)) demand payment or realize on the bond and hold such amount in escrow for the benefit of the plan;
((B)) treat any escrowed payments under this section as if they were plan assets and apply them in a manner consistent with this subtitle; and
((C)) refund any amount to the contributing sponsor which is not required to meet any obligation of the corporation with respect to the plan.
((d)) ** Alternate appropriate procedure** The provisions of this subsection apply in the case of a withdrawal described in subsection (a), and the provisions of subsections (b) and (c) shall not apply, if the corporation determines that the procedure provided for under this subsection is consistent with the purposes of this section and and is more appropriate in the particular case. Upon a showing by the plan administrator of the plan that the withdrawal from the plan by one or more contributing sponsors has resulted, or will result, in a significant reduction in the amount of aggregate contributions to or under the plan, the corporation may—
((1)) require the plan fund to be equitably allocated between those participants no longer working in covered service under the plan as a result of the withdrawal, and those participants who remain in covered service under the plan;
((2)) treat that portion of the plan funds allocable under paragraph (1) to participants no longer in covered service as a plan terminated under ; and
((3)) treat that portion of the plan fund allocable to participants remaining in covered service as a separate plan.
((e)) ** Indemnity agreement** The corporation is authorized to waive the application of the provisions of subsections (b), (c), and (d) of this section whenever it determines that there is an indemnity agreement in effect among contributing sponsors under the plan which is adequate to satisfy the purposes of this section and of .section 1364 of this title