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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 walk(self, filters: str=None, filter_type: type=None, pprint=False, depth=-1):
 """ Iterate tree in pre-order wide-first search order :param filters: filter by python expression :param filter_type: Filter by class :return: """

        children = self.children()
        if children is None:
            children = []
        res = []

        if depth == 0:
            return res
        elif depth != -1:
            depth -= 1

        for child in children:
            if isinstance(child, Formula):
                tmp = child.walk(filters=filters, filter_type=filter_type, pprint=pprint, depth=depth)
                if tmp:
                    res.extend(tmp)

        if filter_type is None:
            if filters is not None:
                if eval(filters) is True:
                    res.append(self)
            else:
                res.append(self)
        elif isinstance(self, filter_type):
            if filters is not None:
                if eval(filters) is True:
                    res.append(self)
            else:
                res.append(self)

        if pprint:
            res = [str(x) + " " for x in res]
            res = "\n".join(res)
        return res