def diag_indices(k=0)
      if ndim < 2
        raise NArray::ShapeError, "must be >= 2-dimensional array"
      end
      m,n = shape[-2..-1]
      NArray.diag_indices(m,n,k)
    end
def diag_indices(k=0)
      if ndim < 2
        raise NArray::ShapeError, "must be >= 2-dimensional array"
      end
      m,n = shape[-2..-1]
      NArray.diag_indices(m,n,k)
    end
Return the k-th diagonal indices.