resource Stack_User()
  import Stack
  var x: Stack.result
  var s1, s2: cap Stack
  var y: int
  s1 := create Stack(10); s2 := create Stack(20)
   ...
  s1.push(4); s1.push(37); s2.push(98)
  if (x := s1.pop(y)) != OK -> ... fi
  if (x := s2.pop(y)) != OK -> ... fi
   ...
end
