Is it ensure, insure, or assure?
Package for Online Programming. Edinburgh, 1966. First of the POP family of languages. Used reverse Polish notation. Implemented as a threaded interpreter. EPU-R-17, U Edinburgh (Jul 1966). "POP-1: An Online Language", R. Popplestone, Mach Intell 2, E. Dale et al eds, Oliver and Boyd, Edinburgh 1968.