code: def infix(self, node, children):
          'infix = "(" expr operator expr ")"'
          _, expr1, operator, expr2, _ = children
          return operator(expr1, expr2f infix(self, node, children):
          'infix = "(" expr operator expr ")"'
          _, expr1, operator, expr2, _ = children
          return operator(expr1, expr2)
docs: infix = "(" expr operator expr ")"