code: def load(*packages):
      
      for pkg in flatten(packages):
          if pkg in loaded_packages:
              logger.debug("skip contrib package '{}', already loaded".format(pkg))
              continue
          loaded_packages.append(pkg)
  
          mod = __import__("law.contrib.{}".format(pkg), globals(), locals(), [pkg])
          logger.debug("loaded contrib package '{}'".format(pkg))
  
          for attr in mod.__all__:
              if hasattr(law, attr):
                  logger.info("cannot register 'law.contrib.{0}.{1}' to 'law.{1}', "
                      "already exists".format(pkg, attr))
              else:
                  setattr(law, attr, getattr(mod, attr))
                  law.__all__.append(attr)
                  logger.debug("registered 'law.contrib.{0}.{1}' to 'law.{1}'".format(pkg, attr))
docs: Loads contrib *packages* and adds members exposed in ``__all__`` to the law main module.
      Example:
  
      .. code-block:: python
  
          import law
          law.contrib.load("numpy")
  
          print(law.NumpyFormatter)
          # -> <class 'law.contrib.numpy.formatter.NumpyFormatter'>
  
      It is ensured that packages are loaded only once.