'Register a *function* that will be applied on the sequence each
time :meth:`record` is called.
:param name: The name of the statistics function as it would appear
in the dictionnary of the statistics object.
:param function: A function that will compute the desired statistics
on the data as preprocessed by the key.
:param argument: One or more argument (and keyword argument) to pass
automatically to the registered function when called,
optional.'

def register(self, name, function, *args, **kargs):

    self.functions[name] = partial(function, *args, **kargs)
    self.fields.append(name)
