for M being non empty set
for H being ZF-formula
for F, G being Function st F is_definable_in M & G is_definable_in M & Free H c= {(x. 3)} holds
for FG being Function st dom FG = M & ( for v being Function of VAR,M holds
( ( M,v |= H implies FG . (v . (x. 3)) = F . (v . (x. 3)) ) & ( M,v |= 'not' H implies FG . (v . (x. 3)) = G . (v . (x. 3)) ) ) ) holds
FG is_definable_in M