reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for XFinSequence;
reserve D for set;

theorem Th17:
  len p <= k & k < len(p^q) implies (p^q).k = q.(k - len p)
proof
  assume that
A1: len p <= k and
A2: k < len(p^q);
  k < len p + len q by A2,Def3;
  hence thesis by A1,Th16;
end;
