code: def argmax(self, values):
          
          keys, maxima = self.max(values)
          maxima = maxima[self.inverse]
          # select the first occurence of the maximum in each group
          index = as_index((self.inverse, values == maxima))
          return keys, index.sorter[index.start[-self.groups:]]
docs: return the index into values corresponding to the maximum value of the group
  
          Parameters
          ----------
          values : array_like, [keys]
              values to pick the argmax of per group
  
          Returns
          -------
          unique: ndarray, [groups]
              unique keys
          argmax : ndarray, [groups]
              index into value array, representing the argmax per group