Intuitionistic Existential Instantiation and Epsilon Symbol
  A natural deduction system for intuitionistic predicate logic with existential \ instantiation rule presented here uses Hilbert's $\e$-symbol. It is conservative over intuitionistic predicate logic. We provide a completeness proof for a suitable Kripke semantics, sketch an approach to a normalization proof, survey related work and state some open problems. Our system extends intuitionistic systems with $\e$-symbol due to A. Dragalin and Sh. Maehara. 
Grigori Mints
math.LO cs.LO