Variables Names With _ Underscores

When the Rust compiler is checking a program to make sure it's valid it also checks for unused variables. TODO: Show an example where the warning for an unused variable shows up.

Adding an _ underscore as the first character of the variable name tells the compiler that we're expecting the variable to be unused so it shoudn't output the warning. This is helpful during the development process to avoid warnings when code isn't fully complete.

We'll be using the underscore in some of the examples to keep the code short without causing the warning.