reserve i,j,k,n,m for Nat,
  x,y,z,y1,y2 for object, X,Y,D for set,
  p,q for XFinSequence;

theorem Th7:
  n < len p implies len (p/^n) = len p -n
proof
  assume n < len p;
  then len p-'n=len p-n by XREAL_0:def 2,XREAL_1:48;
  hence thesis by Def2;
end;
