Have to "return $form" in multi-step form builders

Just spent an hour wondering why my D7 multi-step form doesn't produce any output at all, when the equivalent D6 multi-step form worked just fine.

The form builders must "return $form;" instead of having the form passed by reference:

<?php
function mymodule_step_form($form, &$form_state) {
// add my fields
return $form;
}

Hope that saves others some time.