print("\n--- Test: User class extensions ---");

class Box {
  var v;
  func init(x) { this.v = x; }
  func get() { return this.v; }
}

extension Box {
  func inc() { this.v = this.v + 1; }
  func label(s) { return s + ":" + toString(this.v); }
}

var b = Box(10);
b.inc();
assert(b.get() == 11, "Box.inc should increment value");
assert(b.label("val") == "val:11", "Box.label should return formatted string");

print("User class extensions: PASS");