theorem
  not y in still_not-bound_in p implies All(x,p) => All(y,p) is valid
proof
  assume not y in still_not-bound_in p;
  then All(x,p) => p is valid & not y in still_not-bound_in All(x,p) by Th5,
CQC_THE1:66;
  hence thesis by CQC_THE1:67;
end;
