The predecessor of Unix, Multics was written in PL&#x2F;1 and was very innovative (modern OS still borrow &quot;new ideas&quot;): <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Multics" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Multics</a>