method Main() {
  var t := Triple(18);
  print t, "\n"; // 54
}

method Triple(x: int) returns (r: int) 
  ensures r == 3 * x
{
  var y := Double(x);
  r := x + y;
}

method Double(x: int) returns (r: int) 
  ensures r == 2 * x
{
  r := x + x;
}