code: def alter_poms(pom_dir, additional_params, repo_url=None, mvn_repo_local=None):
      
      work_dir = os.getcwd()
      os.chdir(pom_dir)
  
      try:
          if repo_url:
              settings_filename = create_mirror_settings(repo_url)
          else:
              settings_filename = None
  
          args = ["mvn", "clean"]
          if mvn_repo_local:
              args.extend(["-s", settings_filename])
          if mvn_repo_local:
              args.append("-Dmaven.repo.local=%s" % mvn_repo_local)
          param_list = additional_params.split(" ")
          args.extend(param_list)
  
          logging.debug("Running command: %s", " ".join(args))
          command = Popen(args, stdout=PIPE, stderr=STDOUT)
          stdout = command.communicate()[0]
          if command.returncode:
              logging.error("POM manipulation failed. Output:\n%s" % stdout)
          else:
              logging.debug("POM manipulation succeeded. Output:\n%s" % stdout)
      finally:
          os.chdir(work_dir)
docs: Runs mvn clean command with provided additional parameters to perform pom updates by pom-manipulation-ext.