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

Lemma eq_fbig (A B : {fset I}) (F G : I -> R) :
A =i B -> (forall x, x \in A -> F x = G x) ->
\big[op/idx]_(i <- A) F i = \big[op/idx]_(i <- B) G i.