code: @Nonnull
      public final Word<I> prefix(int prefixLen) {
          final int length = prefixLen < 0 ? length() + prefixLen : prefixLen;
          return subWord(0, length);
      }
docs: Retrieves a prefix of the given length. If <code>length</code> is negative, then a prefix consisting of all but
  the last <code>-length</code> symbols is returned.
  
  @param prefixLen
  the length of the prefix (may be negative, see above).
  
  @return the prefix of the given length.