code: function parseRelationalExpression() {
          var expr, previousAllowIn;
  
          previousAllowIn = allowIn;
          allowIn = true;
          expr = parseShiftExpression();
          allowIn = previousAllowIn;
  
          if (match('<') || match('>') || match('<=') || match('>=')) {
              expr = {
                  type: Syntax.BinaryExpression,
                  operator: lex().value,
                  left: expr,
                  right: parseRelationalExpression()
              };
          } else if (allowIn && matchKeyword('in')) {
              lex();
              expr = {
                  type: Syntax.BinaryExpression,
                  operator: 'in',
                  left: expr,
                  right: parseRelationalExpression()
              };
          } else if (matchKeyword('instanceof')) {
              lex();
              expr = {
                  type: Syntax.BinaryExpression,
                  operator: 'instanceof',
                  left: expr,
                  right: parseRelationalExpression()
              };
          }
  
          return expr;
      }
docs: 11.8 Relational Operators