Description:
------------
The operator precedence table (http://php.net/manual/en/language.operators.precedence.php) does not contain the `yield` and `yield from` operators. Since they can return a value and can be a part of an expression it is important to know their precedence.

For example:

```
$state = yield from self::removeContacts($state, $command->contacts) ?? $state;
```

This code is apparently equivalent to 

```
$state = yield from (self::removeContacts($state, $command->contacts) ?? $state);
```

which I would not have guessed.