Typing inconsistency

[flohw]

Hi,

I use PHPStan within my project and use your library.
PHPStan reports some errors on ElementAbstract when we use it's derivations such as ElementSignature.

Mainly when chaining methods.

See here: https://phpstan.org/r/42328237-5731-43f3-bff4-ca2408a50694

When removing the @return tag or replacing by @return $this, there will be no error.

Would you mind if I provide a PR with this kind of improvements?

Thanks