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

Lemma eq_fbigl_cond (A B : {fset I}) (P Q : pred I) (F : I -> R) :
[fset x in A | P x] =i [fset x in B | Q x] ->
\big[op/idx]_(i <- A | P i) F i = \big[op/idx]_(i <- B | Q i) F i.