How to enter multiline commands (statements) into the Scala REPL

When you want to test a multiline command/statement in the Scala REPL, you can easily run into a problem where the REPL gets confused, or more accurately, it attempts to evaluate your statement too early.

As a simple example of this, imagine that you want to test the Scala "if" statement syntax. You begin typing your if statement normally, but when you hit [Enter] after your second line, you'll see that everything blows up: