code: def transition(state, symbol)
        dests = @transitions[state][symbol]
        dests = [dests] unless dests.kind_of? Array
        dests
      end
docs: Determines the transition states, if any, from a given
   beginning state and input symbol pair.
  
   @note NFA ε-transitions
     ε-transitions are supported through the use of the
     reserved input alphabet character '&'.
   @param [String] state state label for beginning state.
   @param [String] symbol input symbol.
   @return [Array] Array of destination transition states.