0.1 PSL and Goal-Oriented Conjecturing for Isabelle/HOL
Previously, we developed PSL  for Isabelle/HOL  and its extension to conjecturing mechanism  as initial steps towards the development of a smart proof search in Isabelle . With PSL one can write the following strategy for induction:
strategy DInd = Thens [Dynamic (Induct), Auto, IsSolved]
Dynamic keyword creates variations of the
by specifying different combinations of promising arguments
found in the proof goal and its background proof context.
combines these induction methods with the general purpose proof method,
which checks if there is any proof goal left after applying
PSL keeps applying the combination of
a specialization of
induct method and
auto discharges all remaining sub-goals or
DInd runs out of the variations of
Sometimes it is necessary for human-engineers to come up with auxiliary lemmas, from which they can derive the original goal. To automate this process, we developed a new atomic strategy, Conjecture, as an extension to PSL. Given a proof goal, Conjecture first produces various conjectures that might be useful to discharge the original proof goal, then inserts these conjectures as the premise of the original goal. Thus, for each conjecture, PSL produces two sub-goals: the first sub-goal states that the conjecture implies the original goal, and the second sub-goal states that the conjecture indeed holds. With Conjecture integrated into PSL, one can write the following strategy:
strategy CDInd = Thens [Conjecture, Fastforce, Quickcheck, DInd]
The sequential application of Fastforce prunes conjectures that are not strong enough to prove the original goal, whereas the application of Quickcheck attempts to prune conjectures that are equivalent to False. This way, we can narrow the search space by focusing on promising conjectures; however, when proof goals require many applications of inductions and multiple conjecturing steps, the search space blows up rapidly due to the various induct methods produced by the Dynamic keyword. Since the induct method usually preserves the provability of proof goal, even when the induct method has arguments that are inappropriate to discharge the proof goal, counter-example finders, such as Quickcheck, cannot discard them. To address this problem, we are developing MiLkMaId to suggest how to apply mathematical induction without completing a proof.
0.2 MiLkMaId: Machine Learning Mathematical Induction
The figure below illustrates the overall architecture of MiLkMaId. Similarly to PaMpeR , which suggests promising proof methods for a given proof goal and its underlying context, MiLkMaId tries to learn how to apply induction effectively using human-written proof corpora as training data. In the preparation phase, MiLkMaId collects invocations of the induct
method appearing in the proof corpora and converts each of them into a simpler format, a vector of booleans using an assertion-based feature extractor. Then,MiLkMaId constructs a regression tree , which describes not only which variations of the induct method are promising but also which assertions are useful to make such recommendations in the recommendation phase.
The mechanism of MiLkMaId differs from that of PaMpeR in multiple ways. First of all, MiLkMaId analyzes proof corpora via what we call active mining: MiLkMaId first creates various induct methods with distinct combinations of arguments, applies each of them to the goal, and compares their results. Secondly, the input to MiLkMaId’s assertions are the triples of a goal with its context, the arguments to the induct method, and the sub-goal appearing after applying induct, whereas PaMpeR’s assertions consider only the first two as input. MiLkMaId takes the emerging sub-goals into considerations: Since the application of the induct method alone is not time-consuming, we expect that it is desirable to improve the accuracy of recommendation using the emerging sub-goals even at the cost of the extra time spent by the induct method. Third, MiLkMaId assertions tend to analyze the structures of the triples, while PaMpeR’s assertions tend to focus on the names of constants and types appearing in the proof goal at hand.
We have implemented the active mining mechanism and around 40 assertions. Our preliminary experiment suggests that the feature extractor successfully distills the essence some undesirable combinations of arguments of induct. However, more comprehensive evaluation and further engineering efforts remain as our future work.
This work was supported by the European Regional Development Fund under the project AI & Reasoning (reg. no.CZ.02.1.01/0.0/0.0/15_003/0000466).
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