Let’s recapture some modelling basics:
What does a diagram like (Some Dia) say, in terms of logic?
Next, we want to assume that the diagram shows all R-relationships of its contained elements, and thus we must exclude cases like (b) by
(2) ¬yRx ∧ ¬xRx ∧ ¬yRy
Moreover, we want to express that we have at least two elements by
(3) x ≠ y
Finally we assume the diagram to cover the whole model, i.e. there are no other elements, by
(4) ∀z x=z ∨ y=z
Thus, all together we get
(5) ∃x, y (1) ∧ (2) ∧ (3) ∧ (4)
(5) describes (some Dia) structurally, i.e. up to isomorphism. Additionally, we could give the elements a material meaning, say a certain Person (x) drives (R) a certain Car (y). (see Unambiguous Models)
Describing single structures is usually not enough in modelling. What we want is to model sets of structures like every Person drives at least one car.