callback() ::
  (from :: state_name(), to :: state_name(), event_name(), event_options() ->
     %Maxine.Data{app: term(), options: term(), tmp: term()})

callback_map() :: %{
  entering: cb_listing(),
  leaving: cb_listing(),
  events: cb_listing(),
  index: cb_index()
}

cb_index() :: %{required(cb_name()) => callback()}

cb_listing() :: %{required(name()) => cb_name() | [cb_name()]}

cb_name() :: atom()

event_name() :: atom()

event_options() :: []

groups_map() :: %{required(name()) => [name()]}

name() :: state_name() | event_name() | cb_name()

state_name() :: atom()

t() :: %Maxine.Machine{
  callbacks: callback_map(),
  groups: groups_map(),
  initial: state_name(),
  transitions: transition_map()
}

transition_map() :: %{
  required(event_name()) => %{required(state_name()) => state_name()}
}