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

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