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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_mask(self, tag=None, field=None):
 """Get the mask for all fields which exist in the current bit field. Parameters tag : str Optionally specifies that the mask should only include fields with the specified tag. field : str Optionally specifies that the mask should only include the specified field. Raises ------ ValueError If a field's length or position has not been defined. (e.g. :py:meth:`.assign_fields` has not been called). UnknownTagError If the tag specified using the `tag` argument does not exist. UnavailableFieldError If the field specified using the `field` argument does not exist or is not available. """

        if tag is not None and field is not None:
            raise TypeError("get_mask() takes exactly one keyword argument, "
                            "either 'field' or 'tag' (both given)")

        selected_fields = self._select_by_field_or_tag(tag, field)

        # Build the mask (and throw an exception if we encounter a field
        # without a fixed size/length.
        mask = 0
        for identifier, field in iteritems(selected_fields):
            if field.length is None or field.start_at is None:
                raise ValueError(
                    "Field '{}' does not have a fixed size/position.".format(
                        identifier))
            mask |= ((1 << field.length) - 1) << field.start_at

        return mask