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 Th12:
  A c= B or B in A
proof
  A in B or A = B or B in A by Th10;
  hence thesis by Def2;
end;
