@techreport{RR-04-23,
  abstract = "Recent work by Knapik, Niwinski and Urzczyn [KNU02] has revived interest in the connexions between higher-order grammars and higher-order pushdown automata. Both devices can be viewed as definitions for term trees as well as string languages. In the latter setting we recall the extensive study by Damm [Dam82], and Damm and Goerdt [DG86]. There it was shown that the language of a level-<em>n</em>&nbsp;higher-order grammar is accepted by a level-<em>n</em>&nbsp;higher-order pushdown automaton subject to the restriction of&nbsp;<em>derived types,&nbsp;</em>more recent rebranded as&nbsp;<em>safety.</em>&nbsp;We show that at level 2, if a string language is generated by an unsafe grammar, there is a (level-2, non-deterministic) safe grammar that generates the same language. Thus safety is not a restriction for level-2 string languages.",
  author = "K. Aehlig and J. G. de Miranda and C.-H. L. Ong",
  institution = "Oxford University Computing Laboratory",
  month = "October",
  number = "RR-04-23",
  title = "Safety is not a restriction at level 2 for string languages",
  year = "2004",
}