  A requirements engineering artifact is valid relative to the stakeholders of
the system-to-be if they agree on the content of that artifact. Checking
relative validity involves a discussion between the stakeholders and the
requirements engineer. This paper proposes (i) a language for the
representation of information exchanged in a discussion about the relative
validity of an artifact; (ii) the acceptability condition, which, when it
verifies in a discussion captured in the proposed language, signals that the
relative validity holds for the discussed artifact and for the participants in
the discussion; and (iii) reasoning procedures to automatically check the
acceptability condition in a discussions captured by the proposed language.
