code: def wherein(self, fieldname, collection, negate=False):
          
          if negate:
              return self.mask([elem not in collection
                                for elem in self[fieldname]])
          else:
              return self.mask([elem in collection
                                for elem in self[fieldname]])
docs: .wherein(fieldname, collection, negate=False)
  
          Returns a new DataTable with rows only where the value at
          `fieldname` is contained within `collection`.