docs: 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)