Unix in the 1970s was completely irrelevant to the context of the article. I don't know to what extent we paid separately for the OS, but we had source for at

Unix in the 1970s was completely irrelevant to the context of the article. I don't know to what extent we paid separately for the OS, but we had source for at least MVT/MVS and OS4000, of ones I used, which we didn't later for SunOS.