// use lower_triangular::LowerTriangularMat;
// 
// /// Choleski factorization.
// pub trait Chol<N> {
//     /// Choleski LL* factorization for a symetric definite positive matrix.
//     fn chol(self) -> LowerTriangularMat<N>;
// }
// 
// impl<N> Chol<N> for DMat<N> {
//     fn chol(self) -> LowerTriangularMat<N> {
//     }
// }
// 
// impl<N> Chol<N> for LowerTriangularMat<N> {
//     fn chol(self) -> LowerTriangularMat<N> {
//     }
// }
// 
// impl<N> Chol<N> for UpperTriangularMat<N> {
//     fn chol(self) -> LowerTriangularMat<N> {
//     }
// }
// 
// impl<N> Chol<N> for DiagonalMat<N> {
//     fn chol(self) -> LowerTriangularMat<N> {
//     }
// }