See also:
  http://www.mailcom.com/besm6/poplan.zip (source of tic tac toe game)
  https://retrocomputing.stackexchange.com/questions/1954/do-any-interesting-pop-2-programs-survive
  https://www.cs.otago.ac.nz/staffpriv/ok/pop2.htm
  https://www.cs.otago.ac.nz/staffpriv/ok/pop2.d/POP-10.pdf
  https://www.cs.otago.ac.nz/staffpriv/ok/pop2.d/Poppy_Manual.pdf
  https://www.cs.otago.ac.nz/staffpriv/ok/pop2.d/PEDIT.pdf
  https://www.chilton-computing.org.uk/acd/icf/sigs/p001.htm
  https://www.cs.bham.ac.uk/research/projects/poplog/comp.lang.pop.faq.html
  https://www.cs.bham.ac.uk/research/projects/poplog/doc/pop11.at.sussex.html
  https://dl.acm.org/doi/pdf/10.1007/s001650200009