def Increment(self, delta, fields=None):
    """Increments counter value by a given delta."""
    if delta < 0:
      raise ValueError("Delta should be > 0 (not %d)" % delta)

    key = self._FieldsToKey(fields)
    if self._values in key:
      self._values[key] += delta
    else:
      self._values[key] = delta
dataset/ETHPy150Open google/grr/grr/lib/stats.py/_CounterMetric.Increment/swapped operands of `in`