In generative cause-effect models, the constraint relation assumption excludes cases where a dependency with an undirected relation $X - Y$ arises because two variables are linked by a logical or mathematical formula, rather than a causal relation. When variables are linked by an equivalent mathematical relation, one can be deduced from the other without any notion of an underlying causal system generating one variable from the other. Such deterministic, non-causal dependencies are not considered in the standard generative cause-effect problem setting.