J, Some time ago (about 3 years) NITE sold their derivitives group to Citi (NITE's most lucrative division). If you look at Knight's fillings, you will see they had something crazy like 2 trillion shares sold and not bought back open under one of their columns.
After the Citi buyout, that went away mysteriously.
So, yes, it does happen. Just not as often as people would like.
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