# E4206

Compiler diagnostic name: `must_implement_one`.

The `#must_implement_one` trait attribute requires each implementation to
provide at least one method from the specified set. If the implementation relies
only on defaults for all required methods, the compiler reports this error.

## Erroneous example

```{literalinclude} /sources/error_codes/4206_error/top.mbt
:language: moonbit
```

## Suggestion

Implement at least one of the methods named by `#must_implement_one`.

```{literalinclude} /sources/error_codes/4206_fixed/top.mbt
:language: moonbit
```