def transform(s):
    """Apply three string transformations and return them in a dict.

    Keys:
      "upper" — s.upper()
      "lower" — s.lower()
      "clean" — s stripped of leading/trailing whitespace,
                lowercased, with spaces replaced by underscores

    transform("  Hello World  ") returns:
      {"upper": "  HELLO WORLD  ",
       "lower": "  hello world  ",
       "clean": "hello_world"}
    """
    pass