Question: How to simplify the equation?

Hi all,

it seems to me that Maple is not able to simplify very simple equations, e.g. the one stored in "d" variable in the following example:

> a:=sqrt(2)*cos(x)-cos(y);
                                 1/2
                           a := 2    cos(x) - cos(y)

> d:=-a/(expand(a*sqrt(2)));
                                  1/2
                                 2    cos(x) - cos(y)
                         d := - ----------------------
                                            1/2
                                2 cos(x) - 2    cos(y)

> simplify(d);
                              1/2
                             2    cos(x) - cos(y)
                            -----------------------
                                         1/2
                            -2 cos(x) + 2    cos(y)

Am I doing something wrong? How to force Maple to simplify the equation above?

Thanks in advance.