code: public StringBuilderWriter 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