Negative field numbers

[ggrothendieck]

Can you clarify how negative field numbers work. If I use this file called s.s

ABC;GHI;JKKK;KLLL
JKL;YUI;

We get

choose -f ; -1 < s.s
KLLL
YUI

so I thought maybe it takes the last non-empty field but this gives the empty field

echo a;b;c; | choose -f  ; -1

One other comment is that it seems that -f; does not work (it gives an error Invalid value for '<choices>...'), only -f ; with a space after -f and before the separator works.

I am doing this all on Windows.