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Ignorance or Mistake as to a matter of fact of law is a defense if it negaties a mental state required to establish a material element of the crime, except that if the defendant would be guilty of another crime had the situation been as he believed, then he may be convicted of the offense of which he would be guilty had the situation been as he believed it to be. (A quite different kind of mistake of law, whereby the defendant believes that his
conduct is not proscribed by the criminal law, is generally not a defense ).