Remarkable. + is not a concatenation character in awk, but it does something here. But what?
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I think I figured it out. The + sign forces what follows into numerical context, but also what...
'file2' can be any length, but it actually has to load all of file1 into memory at once! On a 32-bit computer, this may not even be possible even if you have gigs of memory due to 32-bit addresss...