theorem
  not x in still_not-bound_in p implies ( All(x,p => q) is valid iff p
  => All(x, q ) is valid )
proof
  assume not x in still_not-bound_in p;
  then (p => All(x,q)) <=> All(x,p => q) is valid by Th75;
  hence thesis by Lm15;
end;
