Starting with the extended version of the Expression class you implemented in exercise 4, create a function class called Expression Function that turns an expression string into a function object that maps the value of the variable x into its evaluated result. For example, if you call the constructor

Expression Function ("2 * x + 3")

the result should be a function that you can apply to a single argument. Thus, if you call

Expression Function ("2 * x + 3")(7)

the result should be the value of the expression 2*7+3, or 14.