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The English progressive passive (e.g. is being carried) is first attested in the second half of the eighteenth century. The paper offers a new interpretation of this development as integrated into a series of changes which affected be (and have) at this period. It arose not as a combination of progressive and passive constructions but (with the other changes) was a consequence of the reduction of inflection in auxiliaries which followed the loss of thou in informal speech. This is interpreted as a parametric change, for which there is an overt triggering difference in the primary linguistic data, and the account is formalized within HPSG.
Anthony Warner (Fri,) studied this question.