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
  { A } c= bool A
proof
  A in bool A by Def1;
  hence thesis by Lm1;
end;
