theorem
  Vars(p <=> q,V) = Vars(p,V) \/ Vars(q,V)
proof
A1: the_right_side_of p <=> q = q by QC_LANG2:31;
  p <=> q is biconditional & the_left_side_of p <=> q = p by QC_LANG2:31,def 12
;
  hence thesis by A1,Th49;
end;
