I am not understanding the syntax for accessing webform data in the Additional Processing section of Webform (webform-5.x-1.9)

Given that I have a field with a machine readable name of 'comment' I would expect the following to return a value:

$comment= $form_values['comment'];

$comment is always blank no matter what I put in the form.

What am I doing wrong?

The form resides at http://www.sosclubs.net.au/node/25/