theorem Th18: :: HEYTING1:18

for A being set
for B, C being Element of Fin (DISJOINT_PAIRS A)
for c being Element of DISJOINT_PAIRS A st c in B =>> C holds
ex f being Element of Funcs ((DISJOINT_PAIRS A),[:(Fin A),(Fin A):]) st
( f .: B c= C & c = FinPairUnion (B,((pair_diff A) .: (f,(incl (DISJOINT_PAIRS A))))) )