This section describes the structure of the proof of

Abstract. In spite of the remarkable achievements recently obtained in the field of mechanization of formal reasoning, the overall usability of interactive provers does not seem to be sensibly improved since the advent of the “second generation ” of systems, in the mid of the eighties. We try to analyze the reasons of such a slow progress, pointing out the main problems and suggesting some possible research directions. 1

Abstract. We present a version of the HOL Light system that supports undoing definitions in such a way that this does not compromise the soundness of the logic. In our system the code that keeps track of the constants that have been defined thus far has been moved out of the kernel. This means that the kernel now is purely functional. The changes to the system are small. All existing HOL Light developments can be run by the stateless system with only minor changes. The basic principle behind the system is not to name constants by strings, but by pairs consisting of a string and a definition. This means that the data structures for the terms are all merged into one big graph. OCaml – the implementation language of the system – can use pointer equality to establish equality of data structures fast. This allows the system to run at acceptable speeds. Our system is about 1 6 version of HOL Light. th slower than the stateful

fundamental concepts of a topological space. One such notion is defined as follows [1]: A topological space (X, τ) is connected if and ony if X is not the union of two nonvoid separated subsets, where A and B are separated in X if and only if A ∩ B = � and A ∩ B = �. As usual, Y denotes the closure of a subset Y