$$ n !! k    dyadic: Permutations for k out of n elements (in this case k = n)
$$ #s        monadic: number of elements in s
$$ ,,        monadic: expose with space-lf separators
$$ s[n]      index n of s

'Hello' 123 7.9 '•'=>s;
s[s# !! (s#)],,