a reindex_dprod R idx (op : Monoid.com_law idx) (F : gT -> R) :
   \big[op/idx]_(g in G) F g =
      \big[op/idx]_(k in K) \big[op/idx]_(h inH) F (k * h)%g.
Proof.have /mulgmP/misomP[fM /isomP[injf im_f]] := KxH.