I think perhaps you are not realising the negative tone of your comments. There is no way to read "How is one supposed to know this? Reading the documentation?" except sarcasm. No amount of good faith in unrelated comment threads changes this. I believe that's why you're getting downvoted - not because people are easily offended about D, as you seem to believe.

Well, I did not intend my statement to be sarcastic. It was a genuine question. Blame my lack of social skills, or the fact that I am on the spectrum. I was curious about the implementation details, i.e. why "!" (in map), why "a", why "idup", etc. That is not to say I am reluctant to read the documentation, I am more than willing, but I wanted to know the story behind it. I have ideas, but they might be wrong. I do not want to guess when I can have a direct answer from Walter.