I have a tree: branches are domains and leaves are apps;

Here I have:

INSTALLED_APPS = (
    "django.contrib.admin",
    "django.contrib.auth",
    "django.contrib.contenttypes",
    "django.contrib.redirects",
    "django.contrib.sessions",
    "django.contrib.sites",
    "django.contrib.sitemaps",
    "django.contrib.staticfiles",
    "mezzanine.boot",
    "mezzanine.conf",
    "mezzanine.core",
    "mezzanine.generic",
    "mezzanine.blog",
    "mezzanine.forms",
    "mezzanine.pages",
    "mezzanine.galleries",
    "mezzanine.twitter",
    "mezzanine.accounts",
    "mezzanine.mobile",
    "./reinforced-concrete/beams/RC_BE_BS",
)

When I try to syncdb:

Traceback (most recent call last):
  File "manage.py", line 40, in <module>
    execute_manager(settings)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 459, in execute_manager
    utility.execute()
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 382, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 196, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 231, in execute
    self.validate()
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 266, in validate
    num_errors = get_validation_errors(s, app)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/validation.py", line 30, in get_validation_errors
    for (app_name, error) in get_app_errors().items():
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/loading.py", line 158, in get_app_errors
    self._populate()
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/loading.py", line 64, in _populate
    self.load_app(app_name, True)
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/loading.py", line 86, in load_app
    app_module = import_module(app_name)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/importlib.py", line 28, in import_module
    raise TypeError("relative imports require the 'package' argument")
TypeError: relative imports require the 'package' argument

The leaves contain always forms.

Can somebody help?
Thanks by advance.

This line is not a valid Python import. This is where the "relative" error your seeing is coming from. I am not really sure what you are attempting to do, so I cannot provide a good solution for you. Python packages also cannot have dashes in their names, so the python package of "reinforced-concrete" is invalid even if you remove the "./" from the line.