<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_df(self) -> pd.DataFrame: """Convert to pandas dataframe."""

        df = pd.DataFrame(index=RangeIndex(0, self.shape[0], name=None))
        for key in self.keys():
            value = self[key]
            for icolumn, column in enumerate(value.T):
                df['{}{}'.format(key, icolumn+1)] = column
        return df