function open(code) {
    if (code === codes.numberSign) {
      effects.enter(types.characterReferenceMarkerNumeric)
      effects.consume(code)
      effects.exit(types.characterReferenceMarkerNumeric)
      return numeric
    }

    effects.enter(types.characterReferenceValue)
    max = constants.characterReferenceNamedSizeMax
    test = asciiAlphanumeric
    return value(code)
  }
After `&`, at `#` for numeric references or alphanumeric for named
references.

```markdown
> | a&amp;b
      ^
> | a&#123;b
      ^
> | a&#x9;b
      ^
```

@type {State}