Now propagate downwards. On reaching the invertor, start to propagate True. On reaching junction, place finger at choice point and consider each direction in turn. Propagate horizontally first. On reaching and-gate, place finger on it and start to search backwards.
Search reaches input Thirsty.
Look up truth-value, which is False. Return to previous finger. Start to propagate downwards. On reaching and-gate place finger on it and start to search backwards. On reaching invertor, remember to invert next signal found.