Mutating a tuple in a relation doesn't really make sense per se, because a relation is just a subset of the cartesian product of its attribute domains, and only

Mutating a tuple in a relation doesn't really make sense per se, because a relation is just a subset of the cartesian product of its attribute domains, and only value-based semantics apply to sets. What does make sense is identifying a primary key for a relation, deleting a tuple, and inserting another tuple with the same primary key but different values of its other attributes. In practice, of course, this is equivalent to in-place mutation of identity-objects corresponding to unique values of the primary key.