Maxima assumes order of differentiation assumed to be irrelevant

Hi,

Maxima assumes that the order of differentiation is irrelevant as the
following shows:

(%i1) depends([U],[x,y])$      

(%i2) diff(U,x,1,y,1);
                                       2
                                      d U
(%o2)                                -----
                                     dx dy
(%i3) diff(U,y,1,x,1);
                                       2
                                      d U
(%o3)                                -----
                                     dx dy

Is there any way to override this?

Thanks,

Bernard Hurley.