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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 add_existence(self, rev):
 """Add existence constraint for the field. This is necessary because the normal meaning of 'x > 0' is: x > 0 and is present. Without the existence constraint, MongoDB will treat 'x > 0' as: 'x' > 0 *or* is absent. Of course, if the constraint is already about existence, nothing is done. :rtype: None """

        if len(self.constraints) == 1 and (
            # both 'exists' and strict equality don't require the extra clause
            self.constraints[0].op.is_exists() or
                self.constraints[0].op.is_equality()):
            return
        value = not rev   # value is False if reversed, otherwise True
        constraint = Constraint(self._field, ConstraintOperator.EXISTS, value)
        self._existence_constraints.append(constraint)