function sequenceOpen(code) {
    if (
      code === codes.numberSign &&
      size++ < constants.atxHeadingOpeningFenceSizeMax
    ) {
      effects.consume(code)
      return sequenceOpen
    }

    // Always at least one `#`.
    if (code === codes.eof || markdownLineEndingOrSpace(code)) {
      effects.exit(types.atxHeadingSequence)
      return atBreak(code)
    }

    return nok(code)
  }
In opening sequence.

```markdown
> | ## aa
    ^
```

@type {State}