theorem Th24:
  x-term in T
  proof
    x-term in FreeGen(s,X) = (FreeGen T).s c= (the Sorts of T).s
    by PBOOLE:def 2,def 18,MSAFREE:def 15,def 16;
    then x-term is Element of (the Sorts of T).s;
    hence x-term in Union the Sorts of T;
  end;
