A sequence of structured subsets of PL/I
Richard C. Holt, David B. Wortman · ACM SIGCSE Bulletin · 1974
This paper gives a brief introduction to a sequence of subsets of the PL/I language that has been designed for the purpose of teaching introductory computer, programming. The six subsets, called SP/1, SP/2, ..., SP/6, restrict PL/I to language features which the authors feel encourage good programming habits. Each subset introduces new language features while retaining features introduced in earlier subsets.