TODO

- the following grammar gives incomprehensible
  error messages:

    module: function* expr* ;
    expr: BARELITERAL+ ;
    function: DEF NAME expr ENDDEF ;

- conflict resolution leeds to parse errors:
    ABC: a X Y c;
    X: b | ! ;
    Y: b | ;
  cannot parse 'a b b c'.  Fix this!
- check the computation of expected tokens in case of parse errors
- don't give an error for grammars like
    ABC: ( _X | _Y ) b c;
    _X: a;
    _Y: a;
- write a proper manual page
- Extend the format of grammar files to allow for arbitrary
  (i.e. non-string) symbols in the generated parser.  Or allow for an
  alternative, Python-based input format.



TERMINOLOGY:

rule, production rule, grammar rule    --- r, rule
rule index    --- k, key
word, string, sequence
(terminal, nonterminal) symbol, token    --- X
symbols have "replacements", "expansions", "derivations"
tranparent symbol: name starts with '_', doesn't appear in parse tree
input: (type,value,data)    --- x
parser state as a set    --- I, U
item (rule with a dot)    --- item
parser state as a number    --- state