def remove(self, elem):
        """Remove a requirement.

        :param elem: a string or :class:`Requirement` instance.
        """
        if isinstance(elem, text_type):
            for requirement in self:
                if requirement.package == elem:
                    return super(Requirements, self).remove(requirement)
        return super(Requirements, self).remove(elem)
def find_storm_images_3d(
        top_directory_name, radar_source, radar_field_names,
        radar_heights_m_agl, first_spc_date_string, last_spc_date_string):
    """Locates files with 3-D storm-centered radar images.

    D = number of SPC dates in time period (`first_spc_date_string`...
        `last_spc_date_string`)

    :param top_directory_name: See doc for `find_storm_images_2d`.
    :param radar_source: Same.
    :param radar_field_names: List (length F_r) of radar fields.  Each item must
        be accepted by `radar_utils.check_field_name`.
    :param radar_heights_m_agl: numpy array (length H_r) of radar heights
        (metres above ground level).
    :param first_spc_date_string: First SPC date (format "yyyymmdd").  This
        method will locate files from `first_spc_date_string`...
        `last_spc_date_string`.
    :param last_spc_date_string: Same.
    :return: radar_file_name_matrix: numpy array (D x F_r x H_r) of file paths.
    """

    radar_utils.check_data_source(radar_source)
    first_spc_date_unix_sec = time_conversion.spc_date_string_to_unix_sec(
        first_spc_date_string)
    last_spc_date_unix_sec = time_conversion.spc_date_string_to_unix_sec(
        last_spc_date_string)

    if radar_source == radar_utils.GRIDRAD_SOURCE_ID:
        file_dict = storm_images.find_many_files_gridrad(
            top_directory_name=top_directory_name,
            radar_field_names=radar_field_names,
            radar_heights_m_agl=radar_heights_m_agl,
            start_time_unix_sec=first_spc_date_unix_sec,
            end_time_unix_sec=last_spc_date_unix_sec,
            one_file_per_time_step=False, raise_error_if_all_missing=True)
    else:
        file_dict = storm_images.find_many_files_myrorss_or_mrms(
            top_directory_name=top_directory_name, radar_source=radar_source,
            radar_field_names=[radar_utils.REFL_NAME],
            reflectivity_heights_m_agl=radar_heights_m_agl,
            start_time_unix_sec=first_spc_date_unix_sec,
            end_time_unix_sec=last_spc_date_unix_sec,
            one_file_per_time_step=False,
            raise_error_if_all_missing=True, raise_error_if_any_missing=False)

    radar_file_name_matrix = file_dict[storm_images.IMAGE_FILE_NAMES_KEY]
    num_file_times = radar_file_name_matrix.shape[0]

    if radar_source != radar_utils.GRIDRAD_SOURCE_ID:
        radar_file_name_matrix = numpy.reshape(
            radar_file_name_matrix,
            (num_file_times, 1, len(radar_heights_m_agl))
        )

    time_missing_indices = numpy.unique(
        numpy.where(radar_file_name_matrix == '')[0]
    )
    return numpy.delete(
        radar_file_name_matrix, time_missing_indices, axis=0)