Verbal agreement in Iberian-Romance

When traditional Grammar mentions agreement in the Iberian-Romance languages, it just refers to the existing agreement between Subject and Predicate (Agr S↔P) regarding the person (1st, 2nd or 3rd) and the number (singular and plural), but it may also affect gender when dealing with passive constructions. Some Romance languages (French, Italian, Sardinian, Catalan) also know double agreement affecting both Subject and Object (Agr. S↔P↔O), as in Catalan les pomes, pro les he comprades les pomes, where the second agreement affects both gender and number. Yet there is a third type of agreement: Object Agreement (Agr. P↔O), which shares some features with the first type of agreement, since it also refers to both person and number. It is this third type of agreement we will focus on, as it occurs in very specific syntactic contexts (the so-called ergative and impersonal middle voice); it presents exclusive features in the Iberian-Romanian languages (Portugalician, Spanish and Catalan), with a complex syntactic structure requiring from the Diathesis Phrase (DiatP) and a couple of Agreement Phrases (SubjAgP and ObjAgrP) to explain its nature.