Source : Free On-Line Dictionary of Computing

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.