This whole section is targeted at Symfony users (not developers; the dev section is below). So this means that a user should not add a method to an implementation that does not exist in our interface (at least we cannot guarantee BC).

Example:

```php
/** regular interface, not API */
interface SymfonyInterface
{
    public function foo();
}

class MyImplementation implements SymfonyInterface
{
    // BC guaranteed for this method
    public function foo() { ... }

    // BC not guaranteed for this method (if we add bar() to SomeInterface, but with a different signature)
    public function bar() { ... }
}
```