reserve X for set, R,R1,R2 for Relation;
reserve x,y,z for set;
reserve n,m,k for Nat;

theorem Th4:
  [x,y] in R |_2 X iff x in X & y in X & [x,y] in R
  proof
    R |_2 X = X|`(R|X) by WELLORD1:11; then
    [x,y] in R |_2 X iff y in X & [x,y] in R|X by RELAT_1:def 12;
    hence thesis by RELAT_1:def 11;
