WhoKnows offers a powerful search engine with over 20+ filters, boolean logic, and semantic matching logic.  

First, you can access some basic filters at the top of the search page.  To add search criteria, just type in a keyword and hit enter/return or tab.  The search will be refreshed automatically every time a new filter is added.

Secondly, you can click Advanced Search to view even more filters and add more advanced boolean logic.

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