To redefine the whole syntax, one must start with an empty grammar and
empty entries and extend them. The library module
``Grammar.Unsafe'' hold functions to clean up the grammars and
the entries.
Examples can be found in the distribution: the files
``meta/pa_r.ml'' and ``etc/pa_o.ml'' which are the
sources of ``pa_r.cmo'' and ``pa_o.cmo''.