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```/// Defines how the elements of a matrix may be accessed.
pub enum Access<'a, T> {
Dense(&'a dyn DenseAccess<T>),
Sparse(&'a dyn SparseAccess<T>),
}

pub trait Matrix<T> {
fn rows(&self) -> usize;
fn cols(&self) -> usize;

fn access(&self) -> Access<T>;
}

pub trait DenseAccess<T>: Matrix<T> {
fn fetch_single(&self, row: usize, col: usize) -> T;
}

pub trait SparseAccess<T>: Matrix<T> {
/// Number of non-zero elements in the matrix.
fn nnz(&self) -> usize;

/// Retrieve the triplets that identify the coefficients of the sparse matrix.
fn fetch_triplets(&self) -> Vec<(usize, usize, T)>;
}

impl<T, X> Matrix<T> for &X
where
X: Matrix<T>,
{
fn rows(&self) -> usize {
X::rows(*self)
}

fn cols(&self) -> usize {
X::cols(*self)
}

fn access(&self) -> Access<T> {
X::access(*self)
}
}

impl<T, X> DenseAccess<T> for &X
where
X: DenseAccess<T>,
{
fn fetch_single(&self, row: usize, col: usize) -> T {
X::fetch_single(*self, row, col)
}
}

impl<T, X> SparseAccess<T> for &X
where
X: SparseAccess<T>,
{
fn nnz(&self) -> usize {
X::nnz(*self)
}

fn fetch_triplets(&self) -> Vec<(usize, usize, T)> {
X::fetch_triplets(&self)
}
}```