Disprover

ProB Disprover

The ProB Disprover plugin for RODIN utilizes the ProB animator and model checker to automatically find counterexamples for a given problematic proof obligation.

The Disprover is described in Debugging Event-B Models using the ProB Disprover Plug-in, Ligot, Bendisposto, Leuschel.

Installation

The ProB Disprover is available through the ProB Update Site. To Install it:

• Go to the Update manager (Help -> Software Updates...)
• Open the ProB Update Site (http://www.stups.uni-duesseldorf.de/update)
• Select all items under "ProB Disprover"

How to use it, how it works

The proof obligation editor in Rodin presents the user with a number of Hypotheses and one Goal, to be proved. If the Disprover is installed, it is available in the tool bar alongside the other provers.

Upon selecting the Disprover, it builds an Event-B context from the Hypotheses and the Goal (see below). The ProB model checker then tries to set up the constants. If that is possible, this state is a counter example that will be presented back to the user in the proof tree.

Unfortunately, if no counter example is found, there may still be a solution, so we cannot consider the proof to be discharged. Thus, the Disprover will merely mark the proof as "Reviewed".

There are a number of different modes for the Disprover, all with advantages and disadvantages.

The Disprover-Modes

Bild:disprover-relevant-context.png Using relevant Hypotheses and includes all Contexts: Rodin determines which Hypotheses it considers relevant, uses those and includes all Context Information (Sets, Constants, Axioms).

Bild:disprover-all.png Uses all Hypotheses: Simply all Hypotheses are used. As this includes the Axioms from the Contexts as well, including them would be redundant.

Bild:disprover-config.png Allows detailed configuration of the Disprover parameters: A dialog allows the Disprover to be configured in great detail. Warning: If inproperly used, this may result in counter-examples that are not really counter-examples.

Bild:disprover-relevant.png Includes only the relevant hypotheses: Only those hypotheses that Rodin considers relevant are included. This may result in counter-examples that are not really counter-examples, so this mode is discouraged.

Disprover-Preferences

There is an entry for the Disprover in the preferences. These are the same preferences that are presented to the user when running in the configuration mode. These preferences are NOT used for the other modes. When using another use, the ProB-Preferences are used.

The newly constructed context and machine

The generated machine is completely empty (besides an empty INITIALISATION event). It is only generated, because ProB requires it.

The context contains:

• The negated Goals as an Axiom
• The Hypotheses, as Axioms
• The Variables, as Constants
• The Sets
• The Constants

In some modes, those Contexts that are referred from the Sequent are referred through SEES clauses.