theorem Th7:
  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_LANG3:16;
  hence thesis by XBOOLE_0:def 3;
end;
