ID token used for parsing—not part of the lexer.

In [parsing chapter](https://realworldocaml.org/v1/en/html/parsing-with-ocamllex-and-menhir.html) token `ID` is extensively used, however, it is not produced by the [lexer](https://github.com/realworldocaml/examples/blob/master/code/parsing/lexer.mll). This leads to syntax errors while parsing `{}`-style objects.
