Metagrammar description

Grammar file consists of the header with grammar name and rules:

Grammar : grammar str ';' Rules

Rules : Rule                    { [] } 
      | Rules Rule              {  :  }

Rule : id '=' ClauseAlt ';'         { Rule  Nothing   }
     | id ':' id '=' ClauseAlt ';'  { Rule  Nothing   }
     | id '.' id ':' id '=' ClauseAlt ';'  { Rule  (Just )   }
     | '.' id ':' id '=' ClauseAlt ';'  { Rule "String" (Just )   }

ClauseAlt : ClauseAlt1              { Alt (reverse ) }

ClauseAlt1 : ClauseAlt1 '|' ClauseSeq   {  :  } 
           | ClauseSeq                  { [] }

ClauseSeq : ClauseSeq1              { Seq (reverse ) }

ClauseSeq1 : ClauseSeq1 ClausePre    {  :  } 
           | {- empty -}             { [] }

ClausePre : '(' ClauseAlt ')'         {  }
          | ',' ClausePost           { Lifted  }
          | '!' ClausePost           { Ignore  }
          | ClausePost               {  }

ClausePost : ClauseItem '*' OptDelim  { Star   }
           | ClauseItem '+' OptDelim  { Plus   }
           | ClauseItem '?'           { Opt  }
           | ClauseItem               {  }


ClauseItem : id                       { Id  } 
           | str                      { StrLit  }
           | '.'                      { Dot }
           | rexplit                  { RegExpLit  }

OptDelim : {- empty -}          { Nothing }
         | '~' ClauseItem       { Just  }