code: def _frequency(self):
      return "minute"
  
imports: from abc import ABCMeta, abstractmethod, abstractproperty
  from numpy import concatenate
  from lru import LRU
  from pandas import isnull
  from pandas.tslib import normalize_date
  from toolz import sliding_window
  from six import with_metaclass
  from catalyst.assets import Equity, Future
  from catalyst.assets.continuous_futures import ContinuousFuture
  from catalyst.lib._int64window import AdjustedArrayWindow
  from catalyst.lib._float64window import AdjustedArrayWindow
  from catalyst.lib.adjustment import Float64Multiply, Float64Add
  from catalyst.utils.cache import ExpiringCache
  from catalyst.utils.math_utils import number_of_decimal_places
  from catalyst.utils.memoize import lazyval
  from catalyst.utils.numpy_utils import float64_dtype
  from catalyst.utils.pandas_utils import find_in_sorted_index
label_desc: Use image node_id 1 for calling the MinuteHistoryLoader obj's underlying member method code with example usage: obj._frequency() and returns: str