code: def ema(f, c, p=20):
      r"""Calculate the mean on a rolling basis.
  
      Parameters
      ----------
      f : pandas.DataFrame
          Dataframe containing the column ``c``.
      c : str
          Name of the column in the dataframe ``f``.
      p : int
          The period over which to calculate the rolling mean.
  
      Returns
      -------
      new_column : pandas.Series (float)
          The array containing the new feature.
  
      References
      ----------
      *An exponential moving average (EMA) is a type of moving average
      that is similar to a simple moving average, except that more weight
      is given to the latest data* [IP_EMA]_.
  
      .. [IP_EMA] http://www.investopedia.com/terms/e/ema.asp
  
      """
      new_column = pd.ewma(f[c], span=p)
      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 13 for calling a global function with example usage: ema(f, c, p) and returns: new_column