function start(code) {
    assert(code === codes.leftSquareBracket, 'expected `[`')
    effects.enter(type)
    effects.enter(markerType)
    effects.consume(code)
    effects.exit(markerType)
    effects.enter(stringType)
    return atBreak
  }
Start of label.

```markdown
> | [a]
    ^
```

@type {State}