I recently created an open source plugin for Jekyll, the Ruby static site generator. Jekyll LilyPond Converter (henceforth JLC; source, RubyGems) converts specified code snippets in Markdown blog posts to music images using LilyPond. (A small demo exists here.) The project was just complex enough to warrant using a pattern I often use on larger projects, as well as bite me with a sneaky, related “gotcha”, both of which are worth reviewing and sharing. This pattern relates to a concept I call a “seam” that I find myself carefully considering more and more in all the software I write.