callbacks

begin
  old_state       exit                # old? should be deprecated -> use old_state.before_exit instead
  event           before
  old_state       before_exit
  new_state       before_enter
  new_state       enter               # old? should be deprecated -> use new_state.before_enter instead
    ...update state...
    transition    guard
    transition    on_transition
    event         success             # if persist successful
  old_state       after_exit
  new_state       after_enter
  event           after
rescue
  event           error