function start(code) {
    // To do: manually check if interrupting like `markdown-rs`.
    assert(markdownSpace(code))
    effects.enter(types.codeIndented)
    // To do: use an improved `space_or_tab` function like `markdown-rs`,
    // so that we can drop the next state.
    return factorySpace(
      effects,
      afterPrefix,
      types.linePrefix,
      constants.tabSize + 1
    )(code)
  }
Start of code (indented).

> **Parsing note**: it is not needed to check if this first line is a
> filled line (that it has a non-whitespace character), because blank lines
> are parsed already, so we never run into that.

```markdown
> |     aaa
    ^
```

@type {State}