protected Constraints.Constraint collectUpdate() {
      Constraints.Constraint update = Constraints.nilConstraint();
      update = appendFields(update, "$set", setFields);
      update = appendFields(update, "$setOnInsert", setOnInsertFields);
      update = appendFields(update, "$inc", incrementFields);
      update = appendFields(update, "$addToSet", addToSetFields);
      update = appendFields(update, "$push", pushFields);
      update = appendFields(update, "$pull", pullFields);
      update = appendFields(update, "$unset", unsetFields);
      return update;
    }
Returns chained criteria handle used to "OR" new constraint set to form logical DNF.
@return disjunction separated criteria handle
mongo/src/org/immutables/mongo/repository/Repositories.java