Halliday & Matthiessen (2014: 575-6):
Here the transitivity functions remain the same whether the clause is passive or active; there is an exact proportion ants are biting me: I’m getting bitten by ants :: ants keep biting me: I keep getting bitten by ants:
(ants) | keep | = β biting | (me)
(I) | keep | = β getting bitten | (by ants)
no one seems to have mended the lights yet
the lights don’t seem to have been mended yet
when will they start printing the book?
when will the book start being printed?
See Figure 8-6 for the analysis in mood and transitivity.