In this section, we review the main constructs of the Object-Protocol Model (OPM) and the OPM query language (OPM_QL). OPM is an object data model whose non-versioned part is closely related to other semantic [9] and object data models [1]. The version constructs of OPM follow [14]. OPM is described in [4]. OPM_QL is described in [6].