The "advice" feature lets you add to the existing definition of a function, by wrapping around the original function a "piece of advice".
Each piece of advice receives the original function and arguments; its result is returned to the caller as if from the original function.
A function may have multiple pieces of advice, which are composed by nesting earlier advice within later advice.
(Info-goto-node "(elisp)Advising Functions")
val add : ?docstring:string ‑> ?interactive:string ‑> Core_kernel.Source_code_position.t ‑> 'a Value.Type.t ‑> Symbol.t ‑> (inner:(Value.t list ‑> Value.t) ‑> inner_args:Value.t list ‑> 'a) ‑> for_function:Symbol.t ‑> unitadd ?docstring ?interactive here return_type advice_name f ~for_function
defines function advice_name with body f and adds it as advice on for_function.