 Gold did not give a necessary condition for a class to be identifiable in the limit from text, but Angluin (1980) later provided one (in a result almost but not quite obtained by Wexler and Hamburger 1973). Angluin showed that a class C is text-identifiable iff every language L in C has a finite “telltale” subset T such that if T is also proper subset of some other language in C, that other language is not a proper subset of L. This condition precludes guessing too large a language. Once all the members of the telltale subset for L have been received as input, the learner can safely make L the current conjecture. The language to be identified must be either L or (if subsequent inputs include new sentences not in L) some larger language, but it can’t be a proper subset of L.