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diff --git a/src/hydrilla/builder/piggybacking.py b/src/hydrilla/builder/piggybacking.py
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+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# Handling of software packaged for other distribution systems.
+#
+# This file is part of Hydrilla
+#
+# Copyright (C) 2022 Wojtek Kosior
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+#
+#
+# I, Wojtek Kosior, thereby promise not to sue for violation of this
+# file's license. Although I request that you do not make use this code
+# in a proprietary program, I am not going to enforce this in court.
+
+"""
+This module contains definitions that may be reused by multiple piggybacked
+software system backends.
+"""
+
+# Enable using with Python 3.7.
+from __future__ import annotations
+
+from pathlib import Path, PurePosixPath
+from typing import Optional, Iterable
+
+from .. import util
+from .common_errors import *
+
+here = Path(__file__).resolve().parent
+
+_ = util.translation(here / 'locales').gettext
+
+class Piggybacked:
+    """
+    Store information about foreign resources in use.
+
+    Public attributes:
+        'resource_must_depend' (read-only)
+        'package_license_files' (read-only)
+    """
+    def __init__(self, archives: dict[str, Path]={}, roots: dict[str, Path]={},
+                 package_license_files: list[PurePosixPath]=[],
+                 resource_must_depend: list[dict]=[]):
+        """
+        Initialize this Piggybacked object.
+
+        'archives' maps piggybacked system names to directories that contain
+        package(s)' archive files. An 'archives' object may look like
+        {'apt': PosixPath('/path/to/dir/with/debs/and/tarballs')}.
+
+        'roots' associates directory names to be virtually inserted under
+        Hydrilla source package directory with paths to real filesystem
+        directories that hold their desired contents, i.e. unpacked foreign
+        packages.
+
+        'package_license_files' lists paths to license files that should be
+        included with the Haketilo package that will be produced. The paths are
+        to be resolved using 'roots' dictionary.
+
+        'resource_must_depend' lists names of Haketilo packages that the
+        produced resources will additionally depend on. This is meant to help
+        distribute common licenses with a separate Haketilo package.
+        """
+        self.archives              = archives
+        self.roots                 = roots
+        self.package_license_files = package_license_files
+        self.resource_must_depend  = resource_must_depend
+
+    def resolve_file(self, file_ref_name: PurePosixPath) -> Optional[Path]:
+        """
+        'file_ref_name' is a path as may appear in an index.json file. Check if
+        the file belongs to one of the roots we have and return either a path
+        to the relevant file under this root or None.
+
+        It is not being checked whether the file actually exists in the
+        filesystem.
+        """
+        parts = file_ref_name.parts
+        root_path = self.roots.get(parts and parts[0])
+        path = root_path
+        if path is None:
+            return None
+
+        for part in parts[1:]:
+            path = path / part
+
+        path = path.resolve()
+
+        try:
+            path.relative_to(root_path)
+        except ValueError:
+            raise FileReferenceError(_('loading_{}_outside_piggybacked_dir')
+                                     .format(file_ref_name))
+
+        return path
+
+    def archive_files(self) -> Iterable[tuple[PurePosixPath, Path]]:
+        """
+        Yield all archive files in use. Each yielded tuple holds file's desired
+        path relative to the piggybacked archives directory to be created and
+        its current real path.
+        """
+        for system, real_dir in self.archives.items():
+            for path in real_dir.rglob('*'):
+                yield PurePosixPath(system) / path.relative_to(real_dir), path