function beforeSequenceOpen(code) {
    assert(
      code === codes.graveAccent || code === codes.tilde,
      'expected `` ` `` or `~`'
    )

    const tail = self.events[self.events.length - 1]
    initialPrefix =
      tail && tail[1].type === types.linePrefix
        ? tail[2].sliceSerialize(tail[1], true).length
        : 0

    marker = code
    effects.enter(types.codeFenced)
    effects.enter(types.codeFencedFence)
    effects.enter(types.codeFencedFenceSequence)
    return sequenceOpen(code)
  }
In opening fence, after prefix, at sequence.

```markdown
> | ~~~js
    ^
  | alert(1)
  | ~~~
```

@type {State}