theorem Th1:
  X c= V & (o in X implies o is Element of V) & (o in A & A in X
  implies o is Element of V)
proof
  thus X c= V & (o in X implies o is Element of V);
  assume that
A1: o in A and
A2: A in X;
  A c= V by A2,ORDINAL1:def 2;
  hence thesis by A1;
end;
