# M0156

This error means that a parameterized type definition, or set of type definitions, is too complicated for Motoko to accept.

Motoko rejects type definitions that are expansive, in the sense that unfolding type definitions may produce an ever-expanding set of types.

For example, the type definition:

```motoko
type List<T> = ?(T, List<T>);
```

that recursively instantiates `List` at the same parameter `T`, is non-expansive and accepted, but the similar looking definition:

```motoko
type Seq<T> = ?(T, Seq<[T]>);
```

that recursively instantiates `Seq` with a larger type, `[T]`, containing `T`, is *expansive* and rejected.

If you encounter this error, try to restructure your type definitions to be non-expansive.