theorem
  still_not-bound_in p c= still_not-bound_in (All(x,p)) \/ {x}
proof
  set X = (still_not-bound_in p) \ {x};
  still_not-bound_in All(x,p) = X & {x} \/ X = {x} \/ (still_not-bound_in
  p) by QC_LANG3:12,XBOOLE_1:39;
  hence thesis by XBOOLE_1:7;
end;
