docs: def breakpoint(self, name):
          """
          Register the breakpoint of the name. Usage:
          > breakpoint('setup-done')
  
          The name should be used in a call of Driver.register_breakpoint(name)
          method, which returns the Deferred, which will be fired by this
          command.
          """
          if name not in self._breakpoints:
              self.warning("Reached breakpoint %s but found no "
                           "callback registered")
              return
          cb = self._breakpoints[name]
          cb.callback(None)
          return cb