> For example, a method to generate all the permutations of a sequence is easy to get wrong and should be tested whether or not the library using it exposes it.

> For example, a method to generate all the permutations of a sequence is easy to get wrong and should be tested whether or not the library using it exposes it. There are three possibilities here: 1. If your language or common utility libraries have a permutations() method, you shouldn't be rolling your own permutations() method because one exists in libraries. 2. If you're in an environment that doesn't have built in permutations() you should group these kinds of very generic functions that are hard to get right in to some sort of utility module (in which case it would necessarily already be public). 3. If you're in a language that doesn't have built in permutations() and permutations() is in the class which uses it, you have a very generic function on a more specific class, where it has no business being, so it should be moved to a utility class. In all three cases, the solution isn't just "make it public". If you find that you're just making something public to unit test it, this usually points to a much larger problem with your design.