In T 583/93 (OJ 1996, 496) the board held that any attempt to interpret Art. 123(2) EPC 1973 such that the introduction into a claim of features previously described as non‑essential would not be permissible, had to fail. The Convention contained no requirement that forbade the redefinition of an invention provided that Art. 123(2) and (3) EPC 1973 were complied with. Such a redefinition was often necessary in order to take into account prior art not known to the applicant at the priority date. It was therefore possible that features described as optional at the priority date later became essential in the sense that they were necessary to delimit the invention from the prior art. The introduction of such features was permissible provided that, first, the application as originally filed contained an adequate basis for such limitations and, second, the resulting combination of features was still in line with the teaching of the application as originally filed.