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Use different operators to for better search results. Common characters are : and + and - for example
:category1,category2 - search in category1 and category2 (use underscore instead of space in category names)
:category1,category2 keyword - search in category1 and category2, search by keyword
+keyword - must contain keyword
-keyword - must NOT contain keyword
>keyword - give more importance to keyword
<keyword - give less importance to keyword
Odd Insertions 
