The NPL interpreter evaluates the list of generators from left to right so conditions can mention any bound variables that occur to their left. These were known as set comprehensions. NPL eventually evolved into Hope but lost set comprehensions which were called list comprehensions in later functional languages.
Reference: John Darlington, "Program Transformation and Synthesis: Present Capabilities", Research Report No. 77/43, Dept. of Computing and Control, Imperial College of Science and Technology, London September 1977.