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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def to_long_format(df, duration_col):
 """ 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 """

    return df.assign(start=0, stop=lambda s: s[duration_col]).drop(duration_col, axis=1)