 reserve n,m,i,p for Nat,
         h,k,r,r1,r2,x,x0,x1,x2,x3 for Real;
 reserve f,f1,f2,g for Function of REAL,REAL;

theorem
  fD(f,h).x = Shift(f,h).x - f.x
proof
  fD(f,h).x = (fdif(f,h).1).x by DIFF_3:7
    .= Shift(f,h).x - f.x by DIFF_1:11;
  hence thesis;
end;
