let A be QC-alphabet ; :: thesis: for H being Element of QC-WFF A st H is biconditional holds
H = (the_left_side_of H) <=> (the_right_side_of H)
let H be Element of QC-WFF A; :: thesis: ( H is biconditional implies H = (the_left_side_of H) <=> (the_right_side_of H) )
given F, G being Element of QC-WFF A such that A1: H = F <=> G ; :: according to QC_LANG2:def 12 :: thesis: H = (the_left_side_of H) <=> (the_right_side_of H)
the_left_side_of H = F by A1, Th31;
hence H = (the_left_side_of H) <=> (the_right_side_of H) by A1, Th31; :: thesis: verum