My preference would be:
d =: ([ (! <:) +)
and to add a monadic option to the dyadic
d =: d~ : ([ (! <:) +) NB. 4 d 5 ( 4 rolls of 5 sided dice : 70 possible combinations)
I would add the comment including sample arguments and an expected purpose to save me time were I to stumble across it later.
Of course, the final version would be the choice if 0 d 0 were to return 0, even if it does look a little more complicated.