Bug with to_normal_form() function

[vadyushkins]

Hi, @Aunsiels! First of all, thanks a lot for the wonderful library! I was translating the context-free grammar into the Chomsky normal form by using to_normal_form() function and noticed this strange bug.

>>> from pyformlang.cfg import CFG
>>> cfg = CFG.from_text("S -> a S b S")
>>> cnf = cfg.to_normal_form()
>>> cnf.productions
set()

However, I was expecting to see something like this.

S -> S0 S
S0 -> S2 S1
S1 -> b
S2 -> S3 S
S3 -> a

Environment

• OS: Ubuntu 20.04
• Python: 3.8.5
• Pyformlang: 0.1.24, 0.1.23, 0.1.22, 0.1.21, 0.1.20 Yes, on all these versions of the library, the bug is also reproduced.

[vadyushkins]

Sorry, I didn't notice that this grammar specifies an empty language.

[Aunsiels]

No problem :)