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;

theorem Th22:
  len(p^q) = len p + len q
proof
  dom(p^q) = Seg(len p + len q) by Def7;
  hence thesis by Def3;
end;
