macro Swap(a,b,    c)
    typeof(a) c
    c=a
    a=b
    b=c
  end macro


  'demo with compound types:
  '=========================
  type point { float x,y}
  point p={1,2}
  point q={3,4}
  swap p,q
  print "p: "  p.x "," p.y    'p: 3,4
  print "q: "  q.x "," q.y    'q: 1,2