Type systems certify program properties in a compositional way. From a bigger program one can abstract out a part and certify the properties of the resulting abstract program by just using the type of the part that was abstracted away. Termination and productivity are non-trivial yet desired program properties. and several type systems have been put forward that guarantee termination. https://parisnaturalfoodes.shop/product-category/parchment-baking-paper/