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08:45-09:15 Session 37: VSL Opening
Location: EI, EI 7 + EI 9, EI 10 + FH, Hörsaal 1
Welcome Address by the Rector
Welcome Address by the Organizers
SPEAKER: Matthias Baaz
VSL Opening
SPEAKER: Dana Scott
09:15-10:15 Session 38: VSL Keynote Talk
Location: EI, EI 7 + EI 9, EI 10 + FH, Hörsaal 1
VSL Keynote Talk: Computational Ideas and the Theory of Evolution

ABSTRACT. Covertly computational insights have influenced the Theory of Evolution from the very start. I shall discuss recent work on some central problems in Evolution that was inspired and informed by computational ideas. Considerations about the performance of genetic algorithms led to a novel theory of the role of sex in Evolution based on the concept of mixability, the population genetic equations describing the evolution of a species can be reinterpreted as a repeated game between genes played through the multiplicative update algorithm, while a theorem on satisfiability can shed light on the emergence of complex adaptations.

10:15-10:45Coffee Break
10:45-13:00 Session 38C: Verified Theorem Provers
Location: FH, Hörsaal 8
Towards a Formally Verified Proof Assistant
SPEAKER: Vincent Rahli

ABSTRACT. This paper presents a formalization of Nuprl's metatheory in Coq. It includes a nominal-style definition of the Nuprl language, its reduction rules, a coinductive computational equivalence, and a Curry-style type system where a type is defined as a Partial Equivalence Relation (PER) à la Allen. This type system includes Martin-Löf dependent types, a hierarchy of universes, inductive types and partial types. We then prove that the typehood rules of Nuprl are valid w.r.t. this PER semantics and hence reduce Nnuprl's consistency to Coq's consistency.

The reflective Milawa theorem prover is sound (down to the machine code that runs it)
SPEAKER: unknown

ABSTRACT. Milawa is a theorem prover styled after ACL2 but with a small kernel and a powerful reflection mechanism. We have used the HOL4 theorem prover to formalize the logic of Milawa, prove the logic sound, and prove that the source code for the Milawa kernel (2,000 lines of Lisp) is faithful to the logic. Going further, we have combined these results with our previous verification of an x86 machine-code implementation of a Lisp runtime. Our top-level HOL4 theorem states that when Milawa is run on top of our verified Lisp, it will only print theorem statements that are semantically true. We believe that this top-level theorem is the most comprehensive formal evidence of a theorem prover's soundness to date.

HOL with Definitions: Semantics, Soundness, and a Verified Implementation
SPEAKER: Ramana Kumar

ABSTRACT. We present a mechanised semantics and soundness proof for the HOL Light kernel including its definitional principles, extending Harrison's verification of the kernel without definitions. Soundness of the logic extends to soundness of a theorem prover, because we also show that a synthesised implementation of the kernel in CakeML refines the inference system. Our semantics is for Wiedijk's stateless HOL, and is the first for that system; our implementation, however, is stateful, and we give semantics to the stateful inference system by translation to the stateless one. Our model of HOL is parametric on the universe of sets, and thus somewhat cleaner and more general than Harrison's. Additionally, we prove soundness for Arthan's extension of the principle of constant definition, in the hope of encouraging its adoption. This paper represents the logical kernel aspect of our work on verified HOL implementations; the production of a verified machine-code implementation of the whole system with the kernel as a module will appear separately.

Proof Pearl: Proving a Simple Von Neumann Machine Turing Complete

ABSTRACT. In this paper we sketch an ACL2-checked proof that a simple but unbounded Von Neumann machine model is Turing Complete, i.e., can do anything a Turing machine can do. The project formally revisits the roots of computer science. It requires re-familiarizing oneself with the definitive model of computation from the 1930s, dealing with a simple ``modern'' machine model, thinking carefully about the formal statement of an important theorem and the specification of both total and partial programs, writing a verifying compiler, including implementing an X86-like call/return protocol and implementing computed jumps, codifying a code proof strategy, and a little ``creative'' reasoning about the non-termination of two machines.

Rough Diamond: An Extension of Equivalence-based Rewriting
SPEAKER: Matt Kaufmann

ABSTRACT. Previous work by the authors generalized conditional rewriting from the use of equalities to the use of arbitrary equivalence relations. This (classic) {\em equivalence-based rewriting} automates the replacement of one subterm by another that may not be strictly equal to it, but is equivalent to it, where this equivalence is determined {\em automatically} to be sufficient at that subterm occurrence. We extend that capability by introducing {\em patterned} congruence rules in the ACL2 theorem prover, to provide more control over the occurrences where such a replacement may be made. This extension enables additional automation of the rewriting process, which is important in industrial-scale applications. However, because this feature is so new (introduced January, 2014), we do not yet have industrial applications to verify its utility, so we present a small example that illustrates how it supports scaling to large proof efforts.

13:00-14:30Lunch Break
14:30-16:00 Session 41B: Invited Talk
Location: FH, Hörsaal 8
From Operational Models to Information Theory - Side-Channels in pGCL with Isabelle
SPEAKER: David Cock

ABSTRACT. In this paper, we formally derive the probabilistic security predicate (expectation) for a guessing attack against a system with side-channel leakage, modelled in pGCL. Our principal theoretical contribution is to link the process-oriented view, where attacker and system execute particular model programs, and the information-theoretic view, where the attacker solves an optimal-decoding problem, viewing the system as a noisy channel. Our practical contribution is to illustrate the selection of probabilistic loop invariants to verify such security properties, and the demonstration of a mechanical proof linking traditionally distinct domains.

Invited talk: Microcode Verification - Another Piece of the Microprocessor Verification Puzzle
SPEAKER: unknown
16:00-16:30Coffee Break
16:30-18:00 Session 42C: Codatatypes
Location: FH, Hörsaal 8
Truly Modular (Co)datatypes for Isabelle/HOL

ABSTRACT. We extended Isabelle/HOL with a pair of definitional commands for datatypes and codatatypes. They support mutual and nested (co)recursion through well-behaved type constructors, including mixed recursion–corecursion, and are complemented by syntaxes for introducing primitively (co)recursive functions and by a general proof method for reasoning coinductively. As a case study, we ported Isabelle’s Coinductive library to use the new commands, eliminating the need for tedious ad hoc constructions.

Recursive Functions on Codatatypes via Domains and Topologies

ABSTRACT. The usual definition facilities in theorem provers cannot handle all recursive functions on codatatypes; the filter function on lazy lists is a prime counterexample. We present two new ways of directly defining functions like filter by exploiting their dual nature as producers and consumers. Borrowing from domain theory and topology, we define them as a least fixpoint (producer view) and as a continuous extension (consumer view). Both constructions yield proof principles that allow elegant proofs.

Experience Implementing a Performant Category-Theory Library in Coq
SPEAKER: Jason Gross

ABSTRACT. We describe our experience implementing a broad category-theory library in Coq. Category theory and computational performance are not usually mentioned in the same breath, but we have needed substantial engineering effort to teach Coq to cope with large categorical constructions without slowing proof script processing unacceptably. In this paper, we share the lessons we have learned about how to represent very abstract mathematical objects and arguments in Coq and how future proof assistants might be designed to better support such reasoning. One particular encoding trick to which we draw attention allows category-theoretic arguments involving duality to be internalized in Coq's logic with definitional equality. Ours may be the largest Coq development to date that uses the relatively new Coq version developed by homotopy type theorists, and we reflect on which new features were especially helpful.