theorem Th3:
  for k being Nat for S being FCEx-Sequence of Al,k holds
  S.(k+1) is QC-alphabet
proof
  let k be Nat;
  let S be FCEx-Sequence of Al,k;
  0 < 0 + (k + 1);
  then 1 <= k + 1 & k + 1 <= k + 1 by NAT_1:19;
  hence thesis by Def7;
end;
