Listing 15.22 says

def show(e: Expr) = s"${println(f.format(e))}\n\n"

But since "\n\n" is not part of the String passed to function println, it
won't produce the line-spacing between expressions shown on page 306.

The code below seems more accurate:

def show(e: Expr) = println(s"${f.format(e)}\n\n")

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Fixed in 5ed. Tnx.