def transition(state, symbol)
      dests = @transitions[state][symbol]
      dests = [dests] unless dests.kind_of? Array
      dests
    end
def transition(state, symbol)
      dests = @transitions[state][symbol]
      dests = [dests] unless dests.kind_of? Array
      dests
    end
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.