let confirm n =
 List.forall(fun elem -> List.length(elem) = List.length(n))n
let remove n =
 match n with
 |[] -> []
 | x::xs -> xs
let submatrix n = remove(List.fold(fun acc elem -> acc@[{remove elem)][]n)
let rec trace n =
 match n with
 |[] -> 0
 |[[]] -> 0
 |(x::xs)::tail -> x + trace (submatrix n)