reserve X,X1,X2,Y,Y1,Y2 for set, p,x,x1,x2,y,y1,y2,z,z1,z2 for object;
reserve f,g,g1,g2,h for Function,
  R,S for Relation;

theorem
  X c= Y implies (f|X)|Y = f|X & (f|Y)|X = f|X by RELAT_1:73,74;
