docs: def commercial_domains():
      # type: () -> set
      """ Return list of commercial email domains, which means:
      - domain is not public
      - domain is not university
      - it is not personal (more than 1 person using this domain)
      >>> "google.com" in commercial_domains()
      True
      >>> "microsoft.com" in commercial_domains()
      True
      >>> "isri.cs.cmu.edu" in commercial_domains()  # university department
      False
      >>> "jaraco.com" in commercial_domains()  # personal
      False
      """
      dus = domain_user_stats()
      es = "test@" + pd.Series(dus.index, index=dus.index)
      return set(
          dus[~is_public_bulk(es) & ~is_university_bulk(es) & (dus > 1)].index)