(* Square root. *)

Lemma normC_sum_eq1 (I : finType) (P : pred I) (F : I -> C) :
`|\sum_(i | P i) F i| = (\sum_(i | P i) `|F i|) ->
(forall i, P i -> `|F i| = 1) ->
{t : C | `|t| == 1 & forall i, P i -> F i = t}.