Q:

Ansible - combining string with an existing variable to register into another variable

I want to combine a fact set with set_facts with a string variable and register it as a new variable.
I think, we cannot register a new variable in an Ansible playbook. So can I use set_facts again to combine a previous set_fact with an existing variable?
I am not sure about the syntax here.
Here is an example:
- vars:
    list_1: usera,userb,userc

- set_fact:
    list_2: "userd,usere,userf"

Now I want to combine both of the string with comma in between and get a variable value like this:
final_list: usera,userb,userc,userd,usere,userf

A:

set_fact:
  final_list: "{{ list_1 }},{{ list_2 }}"

or use the string concatenation operator
set_fact:
  final_list: "{{ list1 ~ ',' ~ list_2 }}"

