code: def add(self, kind, key, *values):
          
          if kind == "pre":
              procs = self.pre
          elif kind == "post":
              procs = self.post
          else:
              raise ValueError("kind is not 'pre' or 'post'")
          self._check_if_registered(key)
          procs[key] = values
docs: Add processor functions.
  
          Any previous list of processors for `kind` and `key` will be
          overwritten.
  
          Parameters
          ----------
          kind : {"pre", "post"}
          key : str
              A registered key.  Add the functions (in order) to this key's list
              of processors.
          *values : callables
              Processors to add.