def METHOD_NAME(df: Any, col_name: str) -> Any:
    """
    Collapses all columns into a single column named `col_name`.
    Parameters
    ----------
    df: a ``pd.DataFrame``.
    col_name: name of new column to store collapsed data.
    Returns
    -------
    A ``pd.DataFrame`` containing all data collapsed into column `col_name`.
    """
    return df.melt(value_name=col_name).drop("variable", axis=1)