How is the Haskell example not "nested" according to your definition? It contains an "a" and two Trees, just like the Rust example. Nothing is optional. (There

How is the Haskell example not "nested" according to your definition? It contains an "a" and two Trees, just like the Rust example. Nothing is optional. (There might be a Maybe or similar type, possibly hidden behind a type name, but it would still not be optional.)