theorem
  for A being disjoint_valued non-empty MSAlgebra over S
  for a being s-sort Element of A
  holds the_sort_of a = s
  proof
    let A be disjoint_valued non-empty MSAlgebra over S;
    let a be s-sort Element of A;
    a in (the Sorts of A).s by AS;
    hence the_sort_of a = s by SORT;
  end;
