reserve X,Y,Z,X1,X2,X3,X4,X5,X6 for set, x,y for object;
reserve a,b,c for object, X,Y,Z,x,y,z for set;
reserve A,B,C,D for Ordinal;

theorem
  A,B are_c=-comparable
proof
  A c= B or B c= A;
  hence thesis;
end;
