def add_existence(self, rev):
if len(self.constraints) == <NUM_LIT:1> and (<EOL>self.constraints[<NUM_LIT:0>].op.is_exists() or<EOL>self.constraints[<NUM_LIT:0>].op.is_equality()):<EOL><INDENT>return<EOL><DEDENT>value = not rev   <EOL>constraint = Constraint(self._field, ConstraintOperator.EXISTS, value)<EOL>self._existence_constraints.append(constraint)<EOL>
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