reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for FinSequence;
reserve D for set;
reserve a, b, c, d, e, f for object;

theorem
  for p,q being FinSequence st p = p^q or p = q^p holds q = {}
proof
  let p,q be FinSequence such that
A1: p = p^q or p = q^p;
   len(p^q) = len p + len q & len(q^p) = len q + len p by Th22;
  hence thesis by A1;
end;
