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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 find_field(self, field=None, alias=None):
 """ Finds a field by name or alias. :param field: string of the field name or alias, dict of {'alias': field}, or a Field instance :type field: str or dict or Field :returns: The field if it is found, otherwise None :rtype: :class:`Field <querybuilder.fields.Field>` or None """

        if alias:
            field = alias
        field = FieldFactory(field, table=self, alias=alias)
        identifier = field.get_identifier()
        for field in self.fields:
            if field.get_identifier() == identifier:
                return field
        return None