How to use the nitpick.fields.Nested function in nitpick

def file_field_pair(file_name: str, base_file_class: Type[NitpickPlugin]) -> Dict[str, fields.Field]:
        """Return a schema field with info from a config file class."""
        valid_toml_key = TOMLFormat.group_name_for(file_name)
        unique_file_name_with_underscore = slugify(file_name, separator="_")

        kwargs = {"data_key": valid_toml_key}
        if base_file_class.validation_schema:
            field = fields.Nested(base_file_class.validation_schema, **kwargs)
        else:
            # For default files (pyproject.toml, setup.cfg...), there is no strict schema;
            # it can be anything they allow.
            # It's out of Nitpick's scope to validate those files.
            field = fields.Dict(fields.String, **kwargs)
        return {unique_file_name_with_underscore: field}

TEXT_FILE_RTFD_PAGE = "config_files.html#text-files"


class TextItemSchema(Schema):
    """Validation schema for the object inside ``contains``."""

    error_messages = {"unknown": help_message("Unknown configuration", TEXT_FILE_RTFD_PAGE)}
    line = fields.NonEmptyString()


class TextSchema(Schema):
    """Validation schema for the text file TOML configuration."""

    error_messages = {"unknown": help_message("Unknown configuration", TEXT_FILE_RTFD_PAGE)}
    contains = fields.List(fields.Nested(TextItemSchema))


class TextPlugin(NitpickPlugin):
    """Checker for text files.

    To check if ``some.txt`` file contains the lines ``abc`` and ``def`` (in any order):

    .. code-block:: toml

        [["some.txt".contains]]
        line = "abc"

        [["some.txt".contains]]
        line = "def"
    """

"""Validation schema for the ``[nitpick.files]`` section on the style file."""

    error_messages = {"unknown": help_message("Unknown file", "nitpick_section.html#nitpick-files")}

    absent = fields.Dict(fields.NonEmptyString, fields.String())
    present = fields.Dict(fields.NonEmptyString, fields.String())
    # TODO: load this schema dynamically, then add this next field setup_cfg
    setup_cfg = fields.Nested(SetupCfgSchema, data_key=SetupCfgPlugin.file_name)


class NitpickSectionSchema(BaseNitpickSchema):
    """Validation schema for the ``[nitpick]`` section on the style file."""

    minimum_version = fields.NonEmptyString()
    styles = fields.Nested(NitpickStylesSectionSchema)
    files = fields.Nested(NitpickFilesSectionSchema)


class BaseStyleSchema(Schema):
    """Base validation schema for style files. Dynamic fields will be added to it later."""

    error_messages = {"unknown": help_message("Unknown file", "config_files.html")}

    nitpick = fields.Nested(NitpickSectionSchema)

class NitpickSectionSchema(BaseNitpickSchema):
    """Validation schema for the ``[nitpick]`` section on the style file."""

    minimum_version = fields.NonEmptyString()
    styles = fields.Nested(NitpickStylesSectionSchema)
    files = fields.Nested(NitpickFilesSectionSchema)


class BaseStyleSchema(Schema):
    """Base validation schema for style files. Dynamic fields will be added to it later."""

    error_messages = {"unknown": help_message("Unknown file", "config_files.html")}

    nitpick = fields.Nested(NitpickSectionSchema)

"""Validation schema for setup.cfg."""

    error_messages = {"unknown": help_message("Unknown configuration", "nitpick_section.html#comma-separated-values")}

    comma_separated_values = fields.List(fields.String(validate=fields.validate_section_dot_field))


class NitpickFilesSectionSchema(BaseNitpickSchema):
    """Validation schema for the ``[nitpick.files]`` section on the style file."""

    error_messages = {"unknown": help_message("Unknown file", "nitpick_section.html#nitpick-files")}

    absent = fields.Dict(fields.NonEmptyString, fields.String())
    present = fields.Dict(fields.NonEmptyString, fields.String())
    # TODO: load this schema dynamically, then add this next field setup_cfg
    setup_cfg = fields.Nested(SetupCfgSchema, data_key=SetupCfgPlugin.file_name)


class NitpickSectionSchema(BaseNitpickSchema):
    """Validation schema for the ``[nitpick]`` section on the style file."""

    minimum_version = fields.NonEmptyString()
    styles = fields.Nested(NitpickStylesSectionSchema)
    files = fields.Nested(NitpickFilesSectionSchema)


class BaseStyleSchema(Schema):
    """Base validation schema for style files. Dynamic fields will be added to it later."""

    error_messages = {"unknown": help_message("Unknown file", "config_files.html")}

    nitpick = fields.Nested(NitpickSectionSchema)

class NitpickFilesSectionSchema(BaseNitpickSchema):
    """Validation schema for the ``[nitpick.files]`` section on the style file."""

    error_messages = {"unknown": help_message("Unknown file", "nitpick_section.html#nitpick-files")}

    absent = fields.Dict(fields.NonEmptyString, fields.String())
    present = fields.Dict(fields.NonEmptyString, fields.String())
    # TODO: load this schema dynamically, then add this next field setup_cfg
    setup_cfg = fields.Nested(SetupCfgSchema, data_key=SetupCfgPlugin.file_name)


class NitpickSectionSchema(BaseNitpickSchema):
    """Validation schema for the ``[nitpick]`` section on the style file."""

    minimum_version = fields.NonEmptyString()
    styles = fields.Nested(NitpickStylesSectionSchema)
    files = fields.Nested(NitpickFilesSectionSchema)


class BaseStyleSchema(Schema):
    """Base validation schema for style files. Dynamic fields will be added to it later."""

    error_messages = {"unknown": help_message("Unknown file", "config_files.html")}

    nitpick = fields.Nested(NitpickSectionSchema)