Design and implementation of a proof assistant for natural deduction

We discuss the design of a proof assistant for the Formal Logic calculus of Natural Deduction based on didactic considerations. We propose to incorporate several features that are not present in the elsewhere available tools, among them the display and edition of derivations as trees, the use of meta-theorems (derived rules) as lemmas, and the possibility of maintaining a set of draft trees that can be inserted into the main derivation as needed. Our assistant checks the validity of each edition operation performed by the user. It has been implemented for propositional logic and (quite satisfactorily) put into practice in courses of Logic for Software Engineering and Information Systems programs.