Care must be taken to avoid an automatic or rigid application of the ordinary cumulative criteria for determining whether a term should be implied in a written contract to a case where the contract is oral or partly oral or where it is apparent that the parties have never attempted to reduce their agreement to complete written form ... The cases in which those criteria were laid down or accepted as the cumulative ingredients of an overall test were concerned with the question whether a term should be implied in a formal contract which was complete upon its face: see, in particular, B.P. Refinery (Westernport) Pty. Ltd. v. Hastings Shire Council ((1977) 52 A.L.J.R. 20, at p. 26); Secured Income Real Estate (Australia) Ltd. v. St. Martin Investments Pty. Ltd. ((1979)144 C.L.R. 596); Codelfa Construction Pty. Ltd. v. State Rail Authority of N.S.W ((1982) 149 C.L.R. 337). In such cases, the insertion of an additional term effectively involves an alteration to what the parties have formally accepted as the complete written record of the compact between them. As the judgment of Mason J. in Codelfa ((1982) 149 C.L.R., at pp. 345-347) (Stephen and Wilson J. concurring with his Honour's comments on this aspect of the case) clearly indicates, the cumulative criteria formulated or accepted in such cases cannot be automatically applied to cases such as the present where the parties have not attempted to spell out all the terms of their contract but have left most or some of them to be inferred or implied. Where that is so, there is no question of effectively altering the terms in which the parties have seen fit to embody their agreement; the function of a court is, as Lord Wilberforce pointed out in Liverpool City Council v. Irwin ([1977] AC 239, at p. 254): 'simply ... to establish what the contract is, the parties not having themselves fully stated the terms.' In the performance of that function, considerations of what is 'reasonable', 'necessary to give business efficacy to the contract' and 'so obvious that 'it goes without saying '' ... may be of assistance in ascertaining the terms which should properly be implied in the contract between the parties. There will not, however, be the need or the justification for the law to refuse to imply any imputed term which does not clearly satisfy all such requirements. This is particularly so where, as here, the contract has passed from the executory stage and has been executed by one or both parties, (emphasis added)