On page 39 of The TeXbook, Knuth said,
Furthermore, spaces are not ignored after control sequences inside a token list; the ignore-space rule applies only in an input file, during the time that strings of characters are being tokenized.
His words confuse me, under what circumstances does TeX produce a series of tokens with space after a control sequence? The output seems like this
<cs>, space_10, a_11, 3_12, what input can make TeX produce this output?