1  struct A;
       2  struct B;
       3  struct D { struct B *b; };
       4  struct Y;
       5  struct Z { struct Y *y; struct D *d; };
       6  struct Y { struct Z z; };
       7  struct X { struct Y y; };
       8  struct C { struct A *a; struct D d; };
       9  struct B { struct C c; struct D d; };
      10  struct A { struct B b; struct X x; } a;