Every source file should begin with a brief description of the contents of that file.
After that description, every file should start the code itself with a
(defmodule ...) form.
;;;; Variable length encoding for integers and floating point numbers. (defmodule num-encode ...)
It is not necessary to include copyright info in every file as long as the project has a
LICENSE file in its top-level directory. Files which differ in license from that file should get have a copyright notice in their header section.
If you are contributing to a project that has established a convention of adding copyright headers to all files, simply follow that convention.