.. py:function:: create_prj(flow, path)

   
   Return the expression tree for the projection operator.


   :Parameters:

       **flow** : :obj:`EX`
           Input flow of the projection.

       **path** : :obj:`Union`\[:obj:`EX`, :obj:`List`\[:obj:`EX`]]
           Elements of the path, which is either the label or index.

   :Returns:

       :obj:`ET`
           ..













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