It is tedious to type in expressions like "[2, '*', 'x']", We'd like to use the more familiar "2*x". This exercise and the next few will get us there. Remember the PEMDAS rule (Parens first, then Exponentiation, then Multiplication and Division, then Addition and Subtraction)? We're going to use it, over the course of the next few exercises, to turn an expression written as a string of characters into an expression consisting of nested components. In this part, we do what is called tokenization: breaking up the string of characters into pieces called tokens: each is a variable name, a number, or an operator: +, -, *, /, or parentheses. The numbers should be actual numbers, not strings. That is, the string "2" should be transformed to the number 2. Everything else should stay as a string.