Mistake in code in Exercise 9.4

I believe one of the lines of code you gave as the answer to Exercise 9.4 has a small typo in it.

You give a model answer as `length(unique(symbols) == 1)` but that doesn't return a *logical* as needed for the exercise.

I think you just have one of the brackets in the wrong place and it should be `length(unique(symbols)) == 1`.