Question: problem with select ?

hi everyone,i have two questions,
1. i have a problem with select command? 



diff(diff(a[1](x), x), x)+(diff(a[1](x), x))*(diff(a[2](x), x))+(diff(diff(a[1](x), x), x))*(diff(diff(a[2](x), x), x))+diff(a[3](x), x)+(diff(a[1](x), x))*(diff(diff(a[2](x), x), x))+(diff(diff(a[1](x), x), x))*(diff(a[3](x), x))


# how can i select the differentials which are of second order ? without using the function names , for example i do not want to use this :


diff(diff(a[1](x), x), x)+(diff(diff(a[1](x), x), x))*(diff(diff(a[2](x), x), x))+(diff(a[1](x), x))*(diff(diff(a[2](x), x), x))+(diff(diff(a[1](x), x), x))*(diff(a[3](x), x))


#i just want maple to select the second order differentials,without knowing its name # just beacuse it is of second order,how can i do this !?

2. how i can select the parts which their addition of differential oreder,are the same ? for example,the second expression in eq[1] has the addition order of 2,third has 4 and ... how can isolate addition order ? tnx in advance . 


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