Section BigFSet.
Variable (R : Type) (idx : R) (op : Monoid.law idx).
Variable (I : choiceType).

Lemma big_imfset key (h : I -> J) (A : finmempred _)
(G : J -> R) : {in A &, injective h} ->
\big[op/idx]_(j <- imfset key h A) G j =
\big[op/idx]_(i <- enum_finmem A) G (h i).