A layered — like an onion — library for writing text editors. The core module just provides facilities for file i/o, and then on top of that, you might add a module for linting, or syntax colouring the output. And then on top of that, a module that interacted with the user. Any module can use any function from any module underneath it, but because it can't make any guarantees about what it will be called by, it may not use any functions from any modules above it. Every module is compiled to a shared object except for the top level module which is compiled to a binary that links to all modules beneath.