function start(code) {
    assert(code === codes.leftSquareBracket, 'expected `[`')
    effects.enter(types.labelLink)
    effects.enter(types.labelMarker)
    effects.consume(code)
    effects.exit(types.labelMarker)
    effects.exit(types.labelLink)
    return after
  }
Start of label (link) start.

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

@type {State}