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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 dropna(self, subset=None):
 """Remove missing values according to Baloo's convention. Parameters subset : list of str, optional Which columns to check for missing values in. Returns ------- DataFrame DataFrame with no null values in columns. """

        subset = check_and_obtain_subset_columns(subset, self)
        not_nas = [v.notna() for v in self[subset]._iter()]
        and_filter = reduce(lambda x, y: x & y, not_nas)

        return self[and_filter]