code: function parse(r, s) {
          var p = new ParseState(s, 0, []);
          if (!(r instanceof AstRule))
              r = r.ast;
          if (!r.parser(p))
              return null;
          return p && p.nodes ? p.nodes[0] : null;
      }
docs: Returns the root node of the abstract syntax tree created by parsing the rule.