def [](i, j=nil)
      if j
        get(i.to_indices, j.to_indices)
      else
        get(i.to_indices)
      end
    end
def [](i, j=nil)
      if j
        get(i.to_indices, j.to_indices)
      else
        get(i.to_indices)
      end
    end
Get the entry at _i_, _j_. If _j_ is omitted, linear
 addressing is used (that is, _i_ just enumerates all entries
 going down rows first.)

 As indices you can use numbers, int[] arrays, matrices (non-zero elements
 are taken as indices then), and ranges.