Definition:Combinable

Let:


 * $$\mathcal{R}_1 \subseteq S_1 \times T_1$$ be a relation on $$S_1 \times T_1$$;
 * $$\mathcal{R}_2 \subseteq S_2 \times T_2$$ be a relation on $$S_2 \times T_2$$;
 * $$X = S_1 \cap S_2$$.

If $$\mathcal{R}_1$$ and $$\mathcal{R}_2$$ agree on $$X$$, they are said to be combinable.