Nevertheless, a party`s insistence on a fixed term of the contract is not necessarily an unfair labour practice. The NLRB and the courts that re-enforce and enforce their injunctions are not prepared to replace their judgment with that of the parties and will not judge the content of collective agreements (NLRB v. American National Insurance Co., 343 U.p. 395, 72 pp. Ct. 824, 96 L. Ed. 1027 ). Moreover, the use of «economic weapons» such as pressure tactics, pickets and strikes to impose bargaining concessions is not necessarily bad faith bargaining (NLRB v. Insurance Agents` International Union, 361 U.p. 477, 80 pp. Ct.
419, 4 L. Ed. 2d 454 ). While a collective agreement is in force, it can only be amended by mutual voluntary agreement. A change in the duration of the contract must be approved by the labour agency. Judge Potter Stewart added in his agreement that matters «at the heart of corporate control,» such as decisions on «the commitment of investment capital and the fundamental scope of the business,» are not mandatory subjects of negotiation. Collective agreements also include decisions on working time and overtime pay. Trade Union Pro`s collective agreements include, for example, agreements on job difference, travel allowance, sickness benefits, maternity benefits, leave allowances and on-call allowances. Once a provisional agreement has been reached between the employer and the union representatives, each member of the union has the opportunity to vote in favour of acceptance or rejection. .