Robot fixture deadlock

A robot cell avoids gridlock with one rule: every robot must claim fixtures in a fixed global ranking, never doubling back. rank_order lists the distinct fixtures from the one that has to be claimed earliest to the one that has to be claimed latest. trace is the recording of what happened, one line per claim or release, written as "robot|take|fixture" or "robot|drop|fixture". A drop always names a fixture that robot is holding right then, and every fixture in the trace appears in rank_order. A take breaks the rule when the fixture ranks no higher than some fixture the robot is already holding, which includes taking a fixture it holds already. Waiting is not your concern: judge ordering only. Reading the trace top to bottom, return the id of the robot whose take breaks the rule first, or an empty string when nothing does.