# gravity in the X/Y directions (constant)
 set ::gravY 0
 set ::gravY 1

 # current lander velocity in the X/Y directions
 set ::velX 1
 set ::velY 1

 # canvas item id for our lander/taxi rectangle
 set ::taxiID 0

 # main event loop, currently running at 1 Hz (poor refresh, simple logic!)
 proc eventLoop {} {
   # check for the taxi being destroyed
   if {$::taxiID == 0} { return }

   # find the x1 y1 x2 y2 coords for the taxi
   set coords [.c coords taxi]

   # if the taxi is moving up (taking off) or has not moved into the ground (aka hard crash or soft land)
   if {$::velY < 0 || [lindex $coords 3] < [.c cget -height]} {
      # accelerate
      incr ::velX $::gravX
      incr ::velY $::gravY

      # move according to velocity
      .c move taxi $::velX $::velY
   }
   # post next update event
   after 1000 eventLoop
 }
 
 # callback for thruster
 proc thrustUp {} {
   incr ::velY -10
 }
 
 proc thrustLt {} {
   incr ::velX -5
 }
 
 proc thrustRt {} {
   incr ::velX 5
 }
 
 ### GUI
 # main frame
 pack [frame .fVel] -side top

 # current Y velocity 
 pack [label .fVel.lLX -text "X Velocity:"] -side left
 pack [label .fVel.lVX -textvariable velX] -side left
 pack [label .fVel.lVY -textvariable velY] -side right
 pack [label .fVel.lLY -text "Y Velocity:"] -side right

 # main playing area 
 pack [canvas .c] -side top

 # create our 'taxi', a yellow rectangle
 set ::taxiID [.c create rectangle 10 10 20 20 -fill yellow -tags taxi]

 # allow the player to thrust
 bind . <Up>    thrustUp
 bind . <Left>  thrustLt
 bind . <Right> thrustRt

 # start the game event loop 
 eventLoop