= TeX control symbol
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A \b[TeX control symbol] is a <TeX control sequence> formed by an escape followed by one character whose <TeX category code> is not letter. Under ordinary conventions, `\%` is one such command.<TeX by Topic by Victor Eijkhout>[§2.7.1: nonletter after escape]{ref}
For an invented example, define `\def\!{Q}` and then read `\! X`. The resulting replacement starts with `Q`, one ordinary space token and `X`. The nonletter ends the command name immediately; the first following space is processed in state M and survives. Compare `\def\word{Q}` followed by `\word X`, which produces `QX`.
Control space, written as an escape followed by a space, has special handling: it enters state S. Thus a rule that discards every space after every command would predict the wrong result for the `\!` example.<TeX source code by Donald E. Knuth>[Scan a control sequence: state selection for spacer and letter; get_next whitespace cases]{ref}
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