code: public String format(Description d, String format, Object... args) {
      checkNotNull(format);
      checkNotNull(args);
      return descriptionFormatter.format(d) + String.format(format, format(args));
    }
docs: Interprets a printf-style format {@code String} for failed assertion messages. It is similar to
  <code>{@link String#format(String, Object...)}</code>, except for:
  <ol>
  <li>the value of the given <code>{@link Description}</code> is used as the first argument referenced in the format
  string</li>
  <li>each of the arguments in the given array is converted to a {@code String} by invoking
  <code>{@link ToString#toStringOf(Object)}</code>.
  </ol>
  
  @param d the description of the failed assertion, may be {@code null}.
  @param format the format string.
  @param args arguments referenced by the format specifiers in the format string.
  @throws NullPointerException if the format string is {@code null}.
  @return A formatted {@code String}.