code: def to_series(df):
      """
      For use in tests; converts DataFrame's first column to Series.
      """
  
      return df[df.columns[0]]
  
imports: from __future__ import division
  import warnings
  from itertools import cycle
  from matplotlib.pyplot import cm
  import numpy
  import pandas
  from IPython.display import display, HTML
  import empyrical.utils
  from .None import pos
  from .None import txn
label_desc: Use image node_id 14 for calling a global function with example usage: to_series(df) and returns: df