Ethic, Hedge, and Adila, the leader  of the revolution, plot out how they can steal an artifact  called the Node of Power. It’s being used to run a heavily fortified train that runs all around the country, providing supplies  to settlements and facilities. This armored behemoth undergoes a complex and unpredictable unloading procedure— a procedure which is displayed, in detail, on a screen within the engine car. Right means the train will go one  car length forward, and left means the train will go the  same distance backwards. While unloading, the train frequently  moves back and forth, so a typical sequence  might look like this. Also within the engine car is a button  that can only be pressed once. When pressed, it lets down the force  field over the artifact for 10 seconds. The engine car is tiny  and designed for a robot. Of your team, only Hedge can fit. The members of the resistance have  positioned a crane over the train tracks that can pluck the artifact  once it’s exposed. They’ll know when to lower  the crane by sight. But the only way Hedge can determine  the train’s position and know when to lower  the force field is by analyzing the unloading procedure, because he’ll be inside  the windowless engine car. Hedge can’t program himself though, so  it’s up to Ethic to tell him what to do. The artifact is in the car  10 positions behind the one that’s directly under the  crane at the start. What instructions can Ethic give Hedge so that he hits the button  at just the right moment? Here’s a hint to get you started. The key to this problem, as with many  programming challenges, is to reframe the information in a way  that a computer can work with. A computer doesn’t know  what a train is, nor does it need to. It can, however, work with variables. Try making a variable that tracks  the train’s position. How will it change as the train moves? Let’s start by breaking this problem  into two objectives. The first is to know where the train will  be as it carries out its instructions. The second is to hit the button when  the train is in just the right position. For the first objective it’ll help to  think of the train as a big number line. Let’s make 0 the car with the node,  1 the car in front of it, and so on. That means car 10 is under  the crane at the start. When the train moves one car right,  car 9 is under the crane. So a right arrow can be  thought of as “subtract 1.” And when the train moves left  from there, 10 is back under the crane, making a left arrow the same as “add 1." Let’s set our train position variable  to 10, since that’s where we start. We can now use a loop  to read the instructions one at a time, adding or subtracting as we go,  to track which car is under the crane. The nice thing about setting up  the variable this way is that it tells us how far the node  is from the crane. So as soon as the variable hits 0,  Hedge should hit the button. And here’s what happens. Ethic gets into position on the crane  while Hedge rushes off and slips into the engine car unnoticed,  just before the train lurches to life. It rolls 3 cars back. 1 forward,  another 4 back. Then so far forward Ethic loses track  before it reverses once more. When the artifact  finally rolls into position, Adila lowers the crane, hoping Ethic and Hedge got it right. At the last possible moment,  the force field sputters and falls. Ethic swoops in, and lifts the  Node of Power to freedom. When Ethic gives the node  to Hedge for safe keeping, something incredible happens. The artifact shimmers to life  with a vision of the past: When the crystal was unearthed,  no one could make the console inside work. The government put out a call  for people to try their luck with it, one at a time. Ethic loved to figure out what made  things tick, so she signed up. Within moments at the console,  something clicked into place, and she created her first robot. The government hired Ethic as  chief robotics engineer on the spot. Within a year her creations ran almost  every aspect of society, and the nation and its people thrived, no longer needing to toil  in the fields and factories. The vision ends, and Hedge detects  the second artifact in the 198forest, to the southeast. Luckily, the train is going there next, and has just enough reserve  fuel for the trip. Ethic and Hedge smuggle themselves aboard and find a hiding spot  for the long journey ahead.