Basic Calculator, ctd.
An innermost expression is an expression that only contains one operation. For example (5-2) is an innermost expression, while (3*(6+9)) is not.
The key idea is that the leftmost of the innermost expressions is evaluated first.
For example, consider (((6+9)/3*(6-4)). Which one is the leftmost innermost expression? (Take 30 seconds to write it.)