reserve
  X,x,y,z for set,
  k,n,m for Nat ,
  f for Function,
  p,q,r for FinSequence of NAT;
reserve p1,p2,p3 for FinSequence;

theorem Th12:
  for p,q being FinSequence st p in ProperPrefixes q holds len p < len q
by Th11,Th5;
