Exercise: prove (or refute) that this is equivalent to the definition I gave in comment #3:

> That is, it is a subset X of the objects along with a subset Y of the attributes, such that every object in X has all the attributes in Y, and X is maximal in this regard.

> That is, it is a subset X of the objects along with a subset Y of the attributes, such that every object in X has all the attributes in Y, and X is maximal in this regard.