In  many  applications  a  linear  time  ontology  may   not   be
useful.    There   are   many   domains  where  a  reasoning with
partially  ordered  objects  is  important   (e.g.,   distributed
programming, controlling versions of documents, or reasoning with
a  object-hierarchy).  We  have   studied    the    algebra    of
incomplete    constraints    between   such  partially    ordered
entities     and     developed    algorithms    for    constraint
propagation in such a domain.