code: public StringWriter append(CharSequence csq) {
          if (csq == null)
              write("null");
          else
              write(csq.toString());
          return this;
      }
docs: Appends the specified character sequence to this writer.
  
  <p> An invocation of this method of the form <tt>out.append(csq)</tt>
  behaves in exactly the same way as the invocation
  
  <pre>
  out.write(csq.toString()) </pre>
  
  <p> Depending on the specification of <tt>toString</tt> for the
  character sequence <tt>csq</tt>, the entire sequence may not be
  appended. For instance, invoking the <tt>toString</tt> method of a
  character buffer will return a subsequence whose content depends upon
  the buffer's position and limit.
  
  @param  csq
  The character sequence to append.  If <tt>csq</tt> is
  <tt>null</tt>, then the four characters <tt>"null"</tt> are
  appended to this writer.
  
  @return  This writer
  
  @since  1.5