code: def to_long_format(df, duration_col):
      
      return df.assign(start=0, stop=lambda s: s[duration_col]).drop(duration_col, axis=1)
docs: This function converts a survival analysis DataFrame to a lifelines "long" format. The lifelines "long"
      format is used in a common next function, ``add_covariate_to_timeline``.
  
      Parameters
      ----------
      df: DataFrame
          a DataFrame in the standard survival analysis form (one for per observation, with covariates, duration and event flag)
      duration_col: string
          string representing the column in df that represents the durations of each subject.
  
      Returns
      -------
      long_form_df: DataFrame
          A DataFrame with new columns. This can be fed into `add_covariate_to_timeline`
  
      See Also
      --------
      to_episodic_format
      add_covariate_to_timeline