 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
  cD(f,h).x = Shift(f,h/2).x - Shift(f,-h/2).x
proof
  cD(f,h).x = (cdif(f,h).1).x by DIFF_3:16
    .= Shift(f,h/2).x - Shift(f,-h/2).x by DIFF_1:25;
  hence thesis;
end;
