bode not working: sys undefined

[Uwe Dippel]

(I don't seem to be all too lucky with my efforts on octave.  :(  )

I tried one example (rather simple) from
https://prof.hti.bfh.ch/sha1/Octave/index/f/bode.html
a basic Bode diagram.

This is what I get:

bode(sys,[],"y_3", {"u_1","u_4"});
error: `sys' undefined near line 3 column 6
error: evaluating argument list element number 1
error: evaluating argument list element number 1

Google didn't help. Since I am still a newbie, I might missing some 
initial setup, have to do some basic reading. Please tell me where and 
what. I am on 3.2.2 as recommended.
By the way, we did create some data structure as indicated in that site, 
but it resulted in other, different, errors. Matlab has no 'sys' there 
in the syntax.
Octave seems to be not all too compatible after all?

Let's start with this bode problem, though, please!

Uwe