<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def apply(self, state_func=None, stash_func=None, stash='active', to_stash=None):
 """ Applies a given function to a given stash. :param state_func: A function to apply to every state. Should take a state and return a state. The returned state will take the place of the old state. If the function *doesn't* return a state, the old state will be used. If the function returns a list of states, they will replace the original states. :param stash_func: A function to apply to the whole stash. Should take a list of states and return a list of states. The resulting list will replace the stash. If both state_func and stash_func are provided state_func is applied first, then stash_func is applied on the results. :param stash: A stash to work with. :param to_stash: If specified, this stash will be used to store the resulting states instead. :returns: The simulation manager, for chaining. :rtype: SimulationManager """

        to_stash = to_stash or stash

        def _stash_splitter(states):
            keep, split = [], []
            if state_func is not None:
                for s in states:
                    ns = state_func(s)
                    if isinstance(ns, SimState):
                        split.append(ns)
                    elif isinstance(ns, (list, tuple, set)):
                        split.extend(ns)
                    else:
                        split.append(s)
            if stash_func is not None:
                split = stash_func(states)
            if to_stash is not stash:
                keep = states
            return keep, split

        return self.split(_stash_splitter, from_stash=stash, to_stash=to_stash)