function start(code) {
    assert(code === codes.ampersand, 'expected `&`')
    effects.enter(types.characterReference)
    effects.enter(types.characterReferenceMarker)
    effects.consume(code)
    effects.exit(types.characterReferenceMarker)
    return open
  }
Start of character reference.

```markdown
> | a&amp;b
     ^
> | a&#123;b
     ^
> | a&#x9;b
     ^
```

@type {State}