32. The system of claim 30 , wherein adding the new value comprises: identifying a collection of values of a first attribute of a first instance using an identifier of the first instance; and establishing a subset of one or more of the identified values as suitably characterizing the first attribute of the first instance.
35. The system of claim 32 , wherein establishing the subset of values as suitable comprises selecting the subset based at least in part on values in the subset meeting a user-specified constraint.