No idea what is in Input.php

Location is Input.php in the Action Module.
Under Class Input in Method Tag().

Code was originally
$content = $this->gen->{$content}();@ But that makes no sense because $content isn't initialized yet. How can it be used as the method in gen?

So what is the correct answer? POSSIBLE SOLUTION:
$content = $this->gen->{$tagData->contentCat}();