                           2 arrives              2 leaves
   1 arrives      1 leaves     : 3 arrives, waits     :      3 leaves
  |-|-------------|-------|--|--------------|-------------|----> time

    [ server busy ][ idle ][====== server busy =======]

  The clock jumps from one event to the next: the long gaps cost
  nothing, and the server's state changes only at an event.
