uniqueness
for b1, b2 being set st ( for A being set holds ( A in b1 iff ex B being Element of FX st ( B in FX & A = B \(PartUnion (B,R)) ) ) ) & ( for A being set holds ( A in b2 iff ex B being Element of FX st ( B in FX & A = B \(PartUnion (B,R)) ) ) ) holds b1= b2