This can be read as:
“Define a variable called mathFunction, which has a type of ‘a function that takes two Int
values, and returns an Int value.’ Set this new variable to refer to the function called
addTwoInts.”
The addTwoInts function has the same type as the mathFunction variable, and so this
assignment is allowed by Swift’s type-checker.
You can now call the assigned function with the name mathFunction: