theorem Th14:
  for D being set, p being XFinSequence of D, n being Nat
  holds n in dom p iff n+1 in dom XFS2FS p
proof
  let D be set, p be XFinSequence of D, n be Nat;
  hereby
    assume n in dom p;
    then n in dom FS2XFS (XFS2FS p);
    hence n+1 in dom XFS2FS p by Th13;
  end;
  assume n+1 in dom XFS2FS p;
  then n in dom FS2XFS (XFS2FS p) by Th13;
  hence thesis;
end;
