Heinlein's works, all, should have gone into post-U.S.-Copyright-protection status 14 years after his death, current statute and the rhetoric of Jack Valenti not-withstanding. As the status of "fair use" and such is hotly contested and heavily, and expensively, litigated, it is best to only ever use ancient myth or one's own demonstrably original and derived from Fair Use Sources. YMMV || IANYL

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