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

Lemma eq_big_idx idx' (I : finType) i0 (P : pred I) F :
P i0 -> right_id idx' *%M ->
\big[*%M/idx']_(i | P i) F i = \big[*%M/1]_(i | P i) F i.