code: function back (self, cb, cancel) {
    if (!cb) cb = noop
    if (self._back.length === 0 || self._pending) return cb(null)
  
    var previous = self._back.pop()
    var current = self.current()
    load(self, previous, done)
  
    function done (err) {
      if (err) return cb(err)
      if (!cancel) self._forward.push(current)
      self._current = previous
      unload(self, current)
      cb(null)
    }
  }
docs: Goes back to the previous screen If 'cancel' is true, removes the current screen from history If 'cancel' if false, the user can return by going forward