reserve u,v,x,x1,x2,y,y1,y2,z,p,a for object,
        A,B,X,X1,X2,X3,X4,Y,Y1,Y2,Z,N,M for set;

theorem Th63:
  {x,y} \ X = {} iff x in X & y in X
proof
  {x,y} \ X = {} iff {x,y} c= X by XBOOLE_1:37;
  hence thesis by Th31;
end;
