def METHOD_NAME(self):
    """
    Converts MutableDenseNDimArray to Matrix. Can convert only 2-dim array, else will raise error.
    Examples
    ========
    >>> a = MutableSparseNDimArray([1 for i in range(9)], (3, 3))
    >>> b = a.tomatrix()
    >>> b
    Matrix([
    [1, 1, 1],
    [1, 1, 1],
    [1, 1, 1]])
    """
    if self.rank() != 2:
        raise ValueError('Dimensions must be of size of 2')
    mat_sparse = {}
    for key, value in self._sparse_array.items():
        mat_sparse[self._get_tuple_index(key)] = value
    return SparseMatrix(self.shape[0], self.shape[1], mat_sparse)