… when I read that in “abcdefg”, f is the character at index -1, the last character 
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page 521, first bullet. The problem is that the explanation starts with an inclusive range, “abcdefg”[1..3], then switches to excluding ranges. I suggest to use inclusive ranges :
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so “abcdefg”[3..-2] returns “def”, since d is the character at index 3, and f is the character at index -2
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Halfway ranges also work, so “abcdefg”[..-2] (or “abcdefg”[...-1]) returns all but the last character
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and “abcdefg”[3..] returns everything from index 3 on (“abcdefg”[3...] including the last character could be seen as an anomaly)