function start(code) {
    assert(code === codes.exclamationMark, 'expected `!`')
    effects.enter(types.labelImage)
    effects.enter(types.labelImageMarker)
    effects.consume(code)
    effects.exit(types.labelImageMarker)
    return open
  }
Start of label (image) start.

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

@type {State}