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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def unicode_to_hex(unicode_string):
 """ Return a string containing the Unicode hexadecimal codepoint of each Unicode character in the given Unicode string. Return ``None`` if ``unicode_string`` is ``None``. Example:: a => U+0061 ab => U+0061 U+0062 :param str unicode_string: the Unicode string to convert :rtype: (Unicode) str """

    if unicode_string is None:
        return None
    acc = []
    for c in unicode_string:
        s = hex(ord(c)).replace("0x", "").upper()
        acc.append("U+" + ("0" * (4 - len(s))) + s)
    return u" ".join(acc)