A Natural Deduction System for Keisler's Quantification

Labelled deduction systems have been used to present a large class of logics. The purpose of this paper is to show N DQ, a (labelled) natural deduction system for Keisler's logic, and discuss some of its properties. This system is the result of the application of a general framework for dealing with quantifiers in natural deduction. The general use of this framework is briefly outlined.