("A", "B", and "C" are all full copies of the whole. Keep track of the stack! You can't exit a lower-level copy somewhere where there's no external connection. (For example, if you're in C, you can't exit via the green path on the lower left, because then there's nowhere to go.) And you can't exit the entire maze, of course.)
Start at the negative terminal; end at the positive.
(by Mark J. P. Wolf.)