code: function parseTopLevel() {
          skipWhitespace();
          peek();
  
          var expr = parseExpression();
          if (expr) {
              if (lookahead.value === ',' || lookahead.value == 'in' &&
                         expr.type === Syntax.Identifier) {
                  parseInExpression(expr);
              } else {
                  parseFilters();
                  if (lookahead.value === 'as') {
                      parseAsExpression(expr);
                  } else {
                      delegate.createTopLevel(expr);
                  }
              }
          }
  
          if (lookahead.type !== Token.EOF) {
              throwUnexpected(lookahead);
          }
      }
docs: FilterExpression :: Expression Expression Filters