Pleco default dictionary

I have seen this asked before and I think it used to be in the add-ons descriptions, but I cannot for the life of me find the answer now, but what is the source for Pleco's in -house dictionary? I have always thought it is very good. At the same time, I have noticed that it is paired more recently with some very good example sentences, more than when I first got Pleco. I was wondering if there was a change and where they ultimate came from. Thanks.
 
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