theorem Th9:
  y in still_not-bound_in s 'or' h iff y in still_not-bound_in s
  or y in still_not-bound_in h
proof
  still_not-bound_in s 'or' h = still_not-bound_in s \/ still_not-bound_in
  h by QC_LANG3:14;
  hence thesis by XBOOLE_0:def 3;
end;
