             eof = true
             return nil
         }
-        if utf8[index] == 91 { // [
+        if utf8[index] == asciiOpenSquareBracket { // [
             index = utf8.index(after: index)
             if let s = parseExtensionKey() {
-                if index == utf8.endIndex || utf8[index] != 93 {
+                if index == utf8.endIndex || utf8[index] != asciiCloseSquareBracket {
                     throw DecodingError.malformedText
                 }
                 // Skip ]

                return result
            }
            let c = utf8[index]
            if c != 39 && c != 34 {
                return result
            }
            index = utf8.index(after: index)
            if let s = parseQuotedString(terminator: c), let b = decodeBytes(s) {
                result.append(b)
            } else {
                throw DecodingError.malformedText


    // If the next token is the identifier "nan", return true.
    private func skipOptionalNaN() -> Bool {
        return skipOptionalKeyword(bytes:
                                  [asciiLowerN, asciiLowerA, asciiLowerN])
    }

    // If the next token is a recognized spelling of "infinity",
