Relations p is a relation from A to B if pAx B,orp∈P(A×B). p is a relation on S if pSx S. We say p relates x and y if(x,y)Ep.Sometimes we write it as xpy. p is an identity relation if∀x,y）∈p.x=y. 12/40 Relations ρ is a relation from A to B if ρ ⊆ A × B, or ρ ∈ P(A × B). ρ is a relation on S if ρ ⊆ S × S. We say ρ relates x and y if (x, y) ∈ ρ. Sometimes we write it as x ρ y. ρ is an identity relation if ∀(x, y) ∈ ρ. x = y. 12 / 40