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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 _merge_simple_selectors(a, b):
 """Merge two simple selectors, for the purposes of the LCS algorithm below. In practice this returns the more specific selector if one is a subset of the other, else it returns None. """

    # TODO what about combinators
    if a.is_superset_of(b):
        return b
    elif b.is_superset_of(a):
        return a
    else:
        return None