code: def gapdown(f):
      r"""Determine whether or not there has been a gap down.
  
      Parameters
      ----------
      f : pandas.DataFrame
          Dataframe with columns ``open`` and ``close``.
  
      Returns
      -------
      new_column : pandas.Series (bool)
          The array containing the new feature.
  
      References
      ----------
      *A gap is a break between prices on a chart that occurs when the
      price of a stock makes a sharp move up or down with no trading
      occurring in between* [IP_GAP]_.
  
      """
      new_column = f["open"] < f["close"].shift(1)
      return new_column
  
imports: from alphapy.calendrical import biz_day_month
  from alphapy.calendrical import biz_day_week
  from alphapy.globals import NULLTEXT
  from alphapy.globals import BSEP, PSEP, USEP
  from alphapy.variables import vexec
  import itertools
  import logging
  import math
  import numpy
  import pandas
label_desc: Use image node_id 20 for calling a global function with example usage: gapdown(f) and returns: new_column