function open(code) {
    if (asciiAlpha(code)) {
      effects.consume(code)
      return schemeOrEmailAtext
    }

    if (code === codes.atSign) {
      return nok(code)
    }

    return emailAtext(code)
  }
After `<`, at protocol or atext.

```markdown
> | a<https://example.com>b
      ^
> | a<user@example.com>b
      ^
```

@type {State}