Exercise 8.6. Assume that the category codes of plain TEX are in force, except that the characters
^^Mbelong respectively to categories 0, 7, 10, and 11. What tokens are produced from the (rather ridiculous) input line
^^B^^BM^^A^^B^^C^^M^^@\M␣? (Remember that this line is followed by ⟨return⟩, which is
^^M; and recall that
^^@denotes the ⟨null⟩ character, which has category 9 when INITEX begins.)
0 is for escape,
7 is for superscript,
10 is for space, and
11 is for letters.
The answer given is:
(I have used
|cs| to denote the control sequence name "cs".)
Question: Why is
␣10 there!? Firstly,
^^C is a non-letter character that follows
^^A^^B (an escape character followed by a non-letter), and thus goes into state S (not M), and it's clearly written that in such a case, any space character is "simply passed by".