Dynamic Placeholders and IExpandable

There have been a few solutions for making placeholder keys dynamic, to allow for page structures that would traditionally result in placeholder keys becoming duplicated. However, nobody seems to have written about how this can naturally be expanded to allow for actual placeholders to be dynamically created based on the whims of the sublayout at the time (or at least, I couldn’t find any articles when I had a quick look). So, here’s how you do it.