BNF grammar parser

 Marpa::R2 parses any language whose grammar can be written in BNF.
 That includes recursive grammars, ambiguous grammars, infinitely
 ambiguous grammars and grammars with useless or empty productions.
 Marpa does both left- and right-recursion in linear time -- in fact if
 a grammar is in any class currently in practical use, Marpa will parse
 it in linear time.