Usage: BIND <key> META3_CHARACTER
  This function switches to another "alternate" keymap.  That is,
  after this character is hit, the next key hit can have a new
  definition.  This is how the Escape sequences are done.  In
  fact, the escape key's default binding is META1_CHARACTER.  For
  example:
    BIND meta1-[ META3_CHARACTER
  binds this function to ^[[.  You may then bind meta key
  sequences with either of the two following formats:
    BIND ^[[A BACKWARD_HISTORY
  or
    BIND META3-A BACKWARD_HISTORY
  Works just like the META1_CHARACTER and the META2_CHARACTER but the
  default binding is meta1-[   which is very useful for binding arrow
  keys and other escape sequences.
