for F, H being ZF-formula st H is biconditional holds
( ( F = the_left_side_of H implies ex G being ZF-formula st H = F <=> G ) & ( ex G being ZF-formula st H = F <=> G implies F = the_left_side_of H ) & ( F = the_right_side_of H implies ex G being ZF-formula st H = G <=> F ) & ( ex G being ZF-formula st H = G <=> F implies F = the_right_side_of H ) ) by Def37, Def38;