The prologue section contains definitions of Emacs Lisp variables, function and macros that are used in the actions in the grammar rules.
These are copied to the beginning of the generated parser file so that
they precede the definition of the grammar rules actions. You can use
require to get the declarations from other libraries. If you
don't need any Emacs Lisp declarations, you may omit this section.
You may have more than one prologue section, intermixed with other declarations. All prologue sections are copied to the generated parser file in the order they appear in the grammar.