Original sentence: Gold, Language Identification in the Limit, Information and Control, 10(5): 447-474, 1967]] showed that any formal language which has hierarchical structure capable of infinite recursion is unlearnable from positive evidence alone, in the sense that it is impossible to formulate a procedure which will discover with certainty the correct grammar given any arbitrary sequence of positive data in which each utterance occurs at least once.Indeed, a proof by [[E.Mark Gold][E
Edited sentence: Indeed, a proof by E.Mark Gold showed that any formal language which has hierarchical structure capable of infinite recursion is unlearnable from positive evidence alone, in the sense that it is impossible to formulate a procedure which will discover with certainty the correct grammar given any arbitrary sequence of positive data in which each utterance occurs at least once