Martin (1992: 414-5):
A comparison of Subject and Finite functions in ranking non-dependent clauses is outlined in Table 6.20. In the interpersonally congruent version of the text, the constituent assumes responsibility more often than the member. The interpersonal message is no longer 'what I can do for you' but rather 'what you can do'.
Table 6.20. Congruent and metaphorical positioning in texts [6:30] and [6:31] subject & finite in ranking non-dependent clauses metaphorical congruent version [6:30] version [6:31] I have you have may I you may we are we are my responsibility is I must (you) do not you may we should you can your problem may we should
 The Subject and Finite together constitute the Mood element, which functions as the nub of the proposition. Modal responsibility, on the other hand, is carried by the Subject (alone).
 For the reasons provided in the previous post, and as the contrasted elements in the table demonstrate, text [6:31] is not an 'interpersonally congruent version' of text [6:30].
 This again confuses metalanguage: modal responsibility, the semantic function of the Subject of a clause, with language: an instance of 'responsibility' in the text.