reserve x for set,
  R for non empty Poset;
reserve S1 for OrderSortedSign,
  OU0 for OSAlgebra of S1;
reserve s,s1,s2,s3,s4 for SortSymbol of S1;

theorem Th42:
  OSSub(OU0) c= MSSub(OU0)
proof
  let x be object;
  assume x in OSSub(OU0);
  then x is strict MSSubAlgebra of OU0 by Def14;
  hence thesis by MSUALG_2:def 19;
end;
