1  
 2  program test.alpha {
 3      struct One {
 4          two: Two,
 5      }
 6  
 7      struct Two {
 8          three: Three,
 9          four: Four,
10      }
11  
12      struct Three {
13          one: One,
14      }
15  
16      struct Four {
17          one: One,
18      }
19  }