theorem Th50:
  not FirstNotUsed p in UsedILoc p
proof
  consider sil being finite Subset of Int-Locations such that
A1: sil = UsedILoc p \/ {intloc 0} and
A2: FirstNotUsed p = FirstNotIn sil by Def7;
  not FirstNotUsed p in sil by A2,SCMFSA_M:14;
  hence thesis by A1,XBOOLE_0:def 3;
end;
