Cases where we don't really have a category: For me, we really need a 7th category for modal lexical verbs, e.g., I was required to leave (also allowed, permitted, obliged, etc.) I find it hard to say these are auxiliary verbs.
 On the SFL model, examples such as I was required/allowed/permitted/obliged to leave do not involve an auxiliary in a verbal group. Instead, these involve a hypotactic verbal group complex. Halliday & Matthiessen (2004: 513):
 O'Donnell's "I find it hard to say" — like "I cannot believe" — is an example of the logical fallacy known as the argument from personal incredulity.… there is a special set that only exist as causatives, where the meaning is simply that of agency: make, cause, force require, let, allow, permit, etc. These admit of three degrees of modulation …