<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sorted_groupby(df, groupby):
 """ Perform a groupby on a DataFrame using a specific column and assuming that that column is sorted. Parameters df : pandas.DataFrame groupby : object Column name on which to groupby. This column must be sorted. Returns ------- generator Yields pairs of group_name, DataFrame. """

    start = 0
    prev = df[groupby].iloc[start]
    for i, x in enumerate(df[groupby]):
        if x != prev:
            yield prev, df.iloc[start:i]
            prev = x
            start = i
    # need to send back the last group
    yield prev, df.iloc[start:]