code: public StringWriter append(CharSequence csq, int start, int end) {
          CharSequence cs = (csq == null ? "null" : csq);
          write(cs.subSequence(start, end).toString());
          return this;
      }
docs: Appends a subsequence of the specified character sequence to this writer.
  
  <p> An invocation of this method of the form <tt>out.append(csq, start,
  end)</tt> when <tt>csq</tt> is not <tt>null</tt>, behaves in
  exactly the same way as the invocation
  
  <pre>
  out.write(csq.subSequence(start, end).toString()) </pre>
  
  @param  csq
  The character sequence from which a subsequence will be
  appended.  If <tt>csq</tt> is <tt>null</tt>, then characters
  will be appended as if <tt>csq</tt> contained the four
  characters <tt>"null"</tt>.
  
  @param  start
  The index of the first character in the subsequence
  
  @param  end
  The index of the character following the last character in the
  subsequence
  
  @return  This writer
  
  @throws  IndexOutOfBoundsException
  If <tt>start</tt> or <tt>end</tt> are negative, <tt>start</tt>
  is greater than <tt>end</tt>, or <tt>end</tt> is greater than
  <tt>csq.length()</tt>
  
  @since  1.5