theorem Th12:
  for p being QC-formula of A holds still_not-bound_in All(x,p) = (
  still_not-bound_in p) \ {x}
proof
  let p be QC-formula of A;
  set a = All(x,p);
A1: a is universal;
  then the_scope_of a = p & bound_in a = x by QC_LANG1:def 27,def 28;
  hence thesis by A1,Th11;
end;
