Imagine you are climbing a large mountain that has never been climbed before. From the bottom, it is impossible to tell which slopes are passable, because much of the terrain is hidden behind the lower foothills. Your climb involves a great deal of trial and error: routes are tried and abandoned; there is frequent backtracking and traversing. Many of the decisions you take may be based on little other than instinct or good fortune. But eventually you do make it to the summit, and once you are there, the ideal route is apparent. You can look down and see what would have been the best path to
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