Section BigConst.
Variables (R : Type) (idx : R) (op : R -> R -> R).

Lemma big_cat I r1 r2 (P : pred I) F :
\big[*%M/1]_(i <- r1 ++ r2 | P i) F i =
\big[*%M/1]_(i <- r1 | P i) F i * \big[*%M/1]_(i <- r2 | P i) F i.