theorem Th8:
  y in still_not-bound_in s '&' h iff y in still_not-bound_in s or
  y in still_not-bound_in h
proof
  still_not-bound_in s '&' h = still_not-bound_in s \/ still_not-bound_in
  h by QC_LANG3:10;
  hence thesis by XBOOLE_0:def 3;
end;
