The Free On-line Dictionary of Computing (30 December 2018):

POP-2

   Robin POPplestone, Edinburgh, 1967.  An innovative language
   incorporating many of Landin's ideas, including streams,
   closures, and functions as first-class citizens.  ALGOL-like
   syntax.  The first implementation was named Multi-POP, based
   on a REVPOL function written in POP-1, producing the
   reverse-polish form as output.  "POP-2 Papers", R.M.  Burstall
   et al, Oliver & Boyd 1968.  "Programming in POP-2",
   R.M. Burstall et al, Edinburgh U Press 1971.  "POP-2 User's
   Manual", R. Popplestone, Mach Intell 2, E. Dale et al eds,
   Oliver and Boyd, Edinburgh 1968.