struct A { int x }
struct B { int x ; int y }
struct C { int x ; int y ; int z }

/*  C <: B     B <: B     */
/* ---------------------- */
/* (B) -> B  <:  (C) -> B */
B f(B b) {
  return b;
}

B g((C) -> B f) {
  var c = new C {x = 1; y = 2; z = 3};
  return f(c);
}

int program(int argc, string[] args) {
  var b = g(f);
  return b.y;
}