let p, q be Element of QC-WFF ; :: thesis: for V being non empty Subset of QC-variables holds Vars (p <=> q),V = (Vars p,V) \/ (Vars q,V)
let V be non empty Subset of QC-variables ; :: thesis: Vars (p <=> q),V = (Vars p,V) \/ (Vars q,V)
( p <=> q is biconditional & the_left_side_of (p <=> q) = p & the_right_side_of (p <=> q) = q ) by QC_LANG2:47, QC_LANG2:def 12;
hence Vars (p <=> q),V = (Vars p,V) \/ (Vars q,V) by Th60; :: thesis: verum