set of tuples.
    Tuple = finite sequence of elements from
    a predesignated set or collection of sets.
    If the tuples all have the same cardinality k,
    then they are called k-tuples and the relation
    is said to have "arity", "adicity", "valence" k.

2.  Relation in Intension = a property ("intension")
    that is common to all of the elements in a set.
    Nota bene:  Saying that a property is shared by
    all of the elements in a set is different from
    saying that the property is a propert