function atBreak(code) {
    if (code === codes.numberSign) {
      effects.enter(types.atxHeadingSequence)
      return sequenceFurther(code)
    }

    if (code === codes.eof || markdownLineEnding(code)) {
      effects.exit(types.atxHeading)
      // To do: interrupt like `markdown-rs`.
      // // Feel free to interrupt.
      // tokenizer.interrupt = false
      return ok(code)
    }

    if (markdownSpace(code)) {
      return factorySpace(effects, atBreak, types.whitespace)(code)
    }

    // To do: generate `data` tokens, add the `text` token later.
    // Needs edit map, see: `markdown.rs`.
    effects.enter(types.atxHeadingText)
    return data(code)
  }
After something, before something else.

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

@type {State}