code: public CharBuffer append (CharSequence csq, int start, int end) {
          if (csq == null) {
              csq = "null"; //$NON-NLS-1$
          }
          CharSequence cs = csq.subSequence(start, end);
          if (cs.length() > 0) {
              return put(cs.toString());
          }
          return this;
      }
docs: Writes chars of the given {@code CharSequence} to the current position of this buffer, and
  increases the position by the number of chars written.
  
  @param csq the {@code CharSequence} to write.
  @param start the first char to write, must not be negative and not greater than {@code
  csq.length()}.
  @param end the last char to write (excluding), must be less than {@code start} and not
  greater than {@code csq.length()}.
  @return this buffer.
  @exception BufferOverflowException if {@code remaining()} is less than {@code end - start}.
  @exception IndexOutOfBoundsException if either {@code start} or {@code end} is invalid.
  @exception ReadOnlyBufferException if no changes may be made to the contents of this buffer.