code:

// See what possible transitions exist, then execute the earliest
// transitions that can be completed
// Returns the time of the last transition
define method process-aircraft
    (airport :: <airport>, #key time = $midnight)
 => (time :: <time-of-day>)
  format-out("Detailed aircraft schedule for ");
  say(airport);
  format-out("\n\n");
  let sky = airport.sky-above;
  let containers-visited = make(<object-table>);
  let active-transitions = make(<sorted-sequence>,
  value-function: earliest-arrival);

  // We do not have to use return as the name of the exit procedure
  block (done)
    while (#t)
      // Each time through, start by considering every container
      fill!(containers-visited, #f);
      // For every container, see if any vehicles are ready to transition
      // If any are, add transition instances to the active-transitions
      // sorted sequence
      generate-new-transitions(sky, active-transitions,
                               containers-visited);

      // If there are no more transitions, we have completed our task
      if (empty?(active-transitions)) done(); end;
      // Find the earliest transition that can complete, because there is
      // still room available in the destination container
      let transition-index
        = find-key(active-transitions,
                   method (transition)
                     available?(transition.transition-aircraft,
                                transition.to-container,
                                transition.transition-aircraft.direction);
                   end);

      // If none can complete, there is a problem with the simulation
      // This situation should never occur, but is useful for debugging
      // incorrect container configurations
      if (transition-index == #f)
        error("Pending transitions but none can complete.");
      end if;

      // Otherwise, the earliest transition that can complete has been
      // found: Execute the transition
      let transition = active-transitions[transition-index];
      let vehicle = transition.transition-aircraft;
      let vehicle-direction = vehicle.direction;
      move-out-vehicle(vehicle, transition.from-container,
                       vehicle-direction);
      move-in-vehicle(vehicle, transition.to-container, vehicle-direction);

      // This transition is complete; remove it from consideration
      transition.pending? := #f;
      remove!(active-transitions, transition);
      // Compute the actual time of arrival at the next container, and
      // display the message
      time := (transition.earliest-arrival := max(time, transition.earliest-arrival));
      say(transition);
      format-out("\n");
    end while;
  end block;
  time;
end method process-aircraft;