code: @Nonnull
      public static String getNCNamePrefix(CharSequence s) {
          if (s.length() > 1 && s.charAt(0) == '_' && s.charAt(1) == ':') {
              return s.toString();
          }
          int localPartStartIndex = getNCNameSuffixIndex(s);
          if (localPartStartIndex > -1) {
              return s.toString().substring(0, localPartStartIndex);
          } else {
              return s.toString();
          }
      }
docs: utility to get the part of a charsequence that is not the NCName
  fragment.
  
  @param s
  the charsequence to split
  @return the prefix split at the last non-ncname character, or the whole
  input if no ncname is found