$ echo c{ar,at,an}s

$ echo c{a{r,t,n}}s

echo c{a{r,t},an}s

echo c{a{r,t,},a{b,n}}

$ echo c{a{r,t,n},}s
cars cats cans cs

{ list; }
              list is simply executed in the current shell environment.  list must be terminated with a newline or semicolon.  This is known as a group command.  The return status is the exit status of list.  Note that  unlike  the
              metacharacters  (  and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized.  Since they do not cause a word break, they must be separated from list by whitespace or another
              shell metacharacter.

c{a        #beginning string
{r,t,n}    #brace expansion
}s         #ending string

echo c{a{r,t},an}s
c        #beginning string
{        #main brace expansion start
a{r,t}   #first element (nested expansion)
,        #list separator
an       #second element
}        #main brace expansion end
s        #ending string

echo c{a{r,t,},a{b,n}}
c        #beginning string
{        #main brace expansion start
a{r,t,}  #first element (nested expansion)
,        #list separator
a{b,n}   #second element (nested expansion)
}        #main brace expansion end

$ echo $0
/bin/bash

$ echo c{a{r,t,n}}s
c{ar}s c{at}s c{an}s

$ /bin/sh
sh-4.1$ echo c{a{r,t,n}}s
c{ar}s c{at}s c{an}s

sh-4.1$ /bin/dash
$ echo c{a{r,t,n}}s
c{a{r,t,n}}s

$ /bin/tcsh
$ echo c{a{r,t,n}}s
cars cats cans

$ /bin/csh
$ echo c{a{r,t,n}}s
cars cats cans

echo c{a{r,t,n}}s

cars cats cans          # bash 2.05b.0(1)
cars cats cans          # bash 3.1.17(2)
c{ar}s c{at}s c{an}s    # bash 4.1.5(2)
c{ar}s c{at}s c{an}s    # bash 4.2.37(2)

echo c{a{r,t,n}}s

cars cats cans

30-06-2022 17:56:07][~]
[grail@killrazor]$ echo c{a{r,t,n}}s
c{ar}s c{at}s c{an}s

$ echo c{a{r,t,n}}s
c{ar}s c{at}s c{an}s
$ bash --version
GNU bash, version 4.3.48(1)-release (x86_64-suse-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.