module Kleene::DSL

def with_err!(nfa, alphabet = nfa.alphabet)

the error state transitions to itself on any token.
adds and error state to the NFA, create error transitions from all non-error states to the error state on any unhandled token. 
def with_err!(nfa, alphabet = nfa.alphabet)
  error_state = nfa.states.find(&:error?)
  return nfa if error_state
  error_state = State.new_error_state
  nfa.add_state(error_state)
  nfa.states.each do |state|
    tokens_on_outbound_transitions = nfa.transitions_from(state).map(&:token)
    missing_tokens = alphabet - tokens_on_outbound_transitions
    missing_tokens.each do |token|
      nfa.add_transition(token, state, error_state)
    end
  end
  nfa.remove_state(error_state) if nfa.all_transitions.none? {|transition| transition.from == error_state || transition.to == error_state }
  nfa.set_regex_pattern("/#{nfa.regex_pattern}/E")
end