uniqueness
for b1, b2 being set st ( for X being set holds ( X in b1 iff ( X c= D * & ( for a, b being FinSequence of D st a in X & b in X holds len a =len b ) ) ) ) & ( for X being set holds ( X in b2 iff ( X c= D * & ( for a, b being FinSequence of D st a in X & b in X holds len a =len b ) ) ) ) holds b1= b2