This probably shouldn't be the first example, but it's a nice one:

My requirement was

I want a list of all ordered permutations of a given length of a set of tokens. Each token is a single character, and for convenience, they are passed as a string in ascending ASCII order.

For example

permute("abc",2)

should return ` ["aa","ab","ac","ba","bb","bc","ca","cb","cc"] `

and permute("13579",3) should return a list of 125 elements

` ["111","113", ... ,"997","999"] `

Ask the experienced coder to code this up in the language of his or her choice. Then show them this (contributed by castironpi):

def p(a,b): if not b: return [''] return [i+j for i in a for j in p(a,b-1)]