It shall be an unfair labor practice for any labor organization and any employer to enter into any contract or agreement, express or implied, whereby such employer ceases or refrains or agrees to cease or refrain from handling, using, selling, transporting or otherwise dealing in any of the products of any other employer, or to cease doing business with any other person, and any contract or agreement entered into heretofore or hereafter containing such an agreement shall be to such extent unenforcible and void: Provided, That nothing in this subsection shall apply to an agreement between a labor organization and an employer in the construction industry relating to the contracting or subcontracting of work to be done at the site of the construction, alteration, painting, or repair of a building, structure, or other work: Provided further, That for the purposes of this subsection and subsection (b)(4)(B) of this section the terms “any employer”, “any person engaged in commerce or an industry affecting commerce”, and “any person” when used in relation to the terms “any other producer, processor, or manufacturer”, “any other employer”, or “any other person” shall not include persons in the relation of a jobber, manufacturer, contractor, or subcontractor working on the goods or premises of the jobber or manufacturer or performing parts of an integrated process of production in the apparel and clothing industry: Provided further, That nothing in this subchapter shall prohibit the enforcement of any agreement which is within the foregoing exception. [Instructor’s note: Emphasis added.]