code: public static String unwrap(final String str, final String prefix, final String suffix) {
          N.checkArgNotNull(prefix);
          N.checkArgNotNull(suffix);
  
          if (N.isNullOrEmpty(str)) {
              return N.EMPTY_STRING;
          } else if (str.length() - prefix.length() >= suffix.length() && str.startsWith(prefix) && str.endsWith(suffix)) {
              return str.substring(prefix.length(), str.length() - suffix.length());
          } else {
              return str;
          }
      }
docs: <p>
  Unwraps the specified string {@code str} if and only if it's wrapped by the specified {@code prefix} and {@code suffix}
  </p>
  
  <pre>
  N.unwrap(null, "[", "]") -> ""
  N.unwrap("", "[", "]") -> ""
  N.unwrap("[", "[", "]") -> "["
  N.unwrap("]", "[", "]") -> "["
  N.unwrap("[abc]", "[", "]") -> "abc"
  N.unwrap("aaaaa", "aa", "aa") -> "a"
  N.unwrap("aa", "aa", "aa") -> "aa"
  N.unwrap("aaa", "aa", "aa") -> "aaa"
  N.unwrap("aaaa", "aa", "aa") -> ""
  </pre>
  
  @param str
  @param prefix
  @param suffix
  @return