reserve X,Y for set, x,y,z for object, i,j,n for natural number;

theorem Th8:
  for S,E being non empty Signature st E is S-extension
  for a being SortSymbol of S holds a is SortSymbol of E
  proof
    let S,E be non empty Signature;
    assume S is Subsignature of E;
    then the carrier of S c= the carrier of E by INSTALG1:10;
    hence thesis;
  end;
