Fix up for swapmod problem. Basically, you can swapmod a swapped out
experiment (multiple times) or even a preloaded experiment (never been swapped). Well, if the first thing you see in an interval is a swapmod, then you have no idea if the experiment was actually swapped in at that time, and counts as an experiment using its nodes! So, have to go back before the interval and look at the previous stat records. As Mac points out, a better solution might be to store this info in the stat record itself, but I want to think about this first.
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