Section BigComImfset.
Variables (R : Type) (idx : R) (op : Monoid.com_law idx)
(I : choiceType) (J : choiceType) (K : choiceType).

Lemma partition_big_imfset (h : I -> J) (A : {fset I}) (F : I -> R) :
\big[op/idx]_(i <- A) F i =
\big[op/idx]_(j <- [fset h x | x in A]) \big[op/idx]_(i <- A | h i == j) F i.