@extra-string/letters (npm): The string 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'.
Keywords: extra|string|letters|LETTERS|at|compare-function|digits|hex-digits|infix|is-infix|is-prefix|is-suffix|left|lowercase|map-function|prefix|replace-function|right|size|suffix|test-function