code: public void update(final String datum) {
      if ((datum == null) || datum.isEmpty()) { return; }
      final byte[] data = datum.getBytes(UTF_8);
      couponUpdate(coupon(hash(data, DEFAULT_UPDATE_SEED)));
    }
docs: Present the given String as a potential unique item.
  The string is converted to a byte array using UTF8 encoding.
  If the string is null or empty no update attempt is made and the method returns.
  
  <p>Note: About 2X faster performance can be obtained by first converting the String to a
  char[] and updating the sketch with that. This bypasses the complexity of the Java UTF_8
  encoding. This, of course, will not produce the same internal hash values as updating directly
  with a String. So be consistent!  Unioning two sketches, one fed with strings and the other
  fed with char[] will be meaningless.
  </p>
  
  @param datum The given String.