This is funny because I was heavily skeptical yet completely reversed my opinion within 10 minutes. I correctly predicted the results, but I program R professionally. Then I asked my partner who's a non-programmer to read the snippet and her prediction was that the program would stop. That led me to reflect back on your statement. I think the key word is "sharp". I correctly predicted the result, but that's only because I trust lazy-eval heavily. Even then, it's hard to really cement trust in lazy-eval (for me) as it's difficult to mentally map. That's not exactly sharp after all!