discussed Cited "see, e.g." Master Window Cleaning, Inc., Dba Bottom Line Enterprises v. National Labor Relations Board, National Labor Relations Board v. Master Window Cleaning, Inc., Dba Bottom Line Enterprises, Service Employees International Union, Local 1877, Respondent-Intervenor Richmond Recording Corp. v. NLRB, 836 F.2d 289 , 293 (7th Cir.1987); see also LAWI/CSA Consolidators v. Teamsters Local 63, 849 F.2d 1236 , 1239-40 (9th Cir.1988) (whether a bargaining impasse has been reached is essentially a factual question that is reviewed for clear error). 5 The Board has defined an impasse as "that point of time in negotiations when the parties are warranted in assuming that further bargaining would be futile." Patrick & Co., 248 N.L.R.B. 390 , 393 (1980), enforced mem., 644 F.2d 889 (9th Cir.1981).