Notation "\big [ op / idx ]_ ( i < n ) F" :=
  (\big[op/idx]_(i : ordinal n) F) : big_scope.Local Notation "x + y" := (plus x y).Notation "\max_ ( i 'in' A ) F" :=
 (\big[maxn/0%N]_(i in A) F%N) : nat_scope.

Lemma big_tnth I r (P : pred I) F : let r_ := tnth (in_tuple r) in \big[op/idx]_(i <- r | P i) F i = \big[op/idx]_(i < size r | P (r_ i)) (F (r_ i)).