theorem
  Vars(p 'or' q, V) = Vars(p,V) \/ Vars(q,V)
proof
A1: the_right_disjunct_of p 'or' q = q by QC_LANG2:29;
  p 'or' q is disjunctive & the_left_disjunct_of p 'or' q = p by QC_LANG2:29
,def 10;
  hence thesis by A1,Th45;
end;
