code: def recall(y_true, y_score, k=None, value=True):
      
      y_true, y_score = to_float(y_true, y_score)
      top = _argtop(y_score, k)
  
      if not value:
          y_true = 1-y_true
  
      r = np.nan_to_num(y_true[top]).sum()
  
      return r
docs: Returns recall (number of positive examples) in the top k
      If value is False then counts number of negative examples
      TODO: add prop argument to return recall proportion instead of count