function start(code) {
    // Do not interrupt paragraphs (but do follow definitions).
    // To do: do `interrupt` the way `markdown-rs` does.
    // To do: parse whitespace the way `markdown-rs` does.
    effects.enter(types.definition)
    return before(code)
  }
At start of a definition.

```markdown
> | [a]: b "c"
    ^
```

@type {State}