128: Why does Emacs say `Key sequence XXX uses invalid prefix characters'?
  
  A prefix of the character sequence you were trying to bind was already
  bound.  Usually, the sequence is "ESC [", in which case you should
  evaluate this form first:
  
    (define-key esc-map "[" nil)
  
  NOTE: By default, "ESC [" is bound to backward-paragraph, and if you do
  this you will lose this key binding.  For most people, this is not a
  problem.
  
  See question 127.