reserve A for non empty set,
  S for non void non empty ManySortedSign;
reserve x for set;

theorem
  for C be category st C = MSSCat A for o be Object of C holds o is non
  empty non void ManySortedSign
proof
  let C be category;
  assume
A1: C = MSSCat A;
  let o be Object of C;
  reconsider o as Element of MSS_set A by A1,Def1;
  o is non empty non void ManySortedSign;
  hence thesis;
end;
