add page for counter-examples plugin

_fc-plugins/counter-examples.md

+---
+layout: plugin
+title: Counter-Examples
+description: Counter-example generation from failed proof attempts
+key: specifications
+distrib_mode: proprietary
+---
+
+## Overview
+
+The **Counter-Examples** plug-in aims at providing counter-examples from failed
+proof attempts by the [WP](wp.html) plug-in.
+More precisely, it generates a test case for the C function under analysis from
+the model given by the automated theorem prover
+(currently, only Alt-Ergo is supported).
+
+
+## Usage
+
+Here are a few usage examples of the **Counter-Examples** plug-in:
+
+```
+frama-c -wp [WP options] -then -ce-unknown
+```
+
+This will generate counter-examples for any annotations marked with *Unknown*
+status by WP.
+
+```
+frama-c -eva [Eva options] -then -ce-alarms
+```
+
+This will try to generate a counter-example for each alarm generated by Eva.
+Note, however, that in the absence of additional annotations, such as function
+contracts and loop invariants, the context in which the counter-example is
+searched for will be too broad, and the generated counter-examples might be
+irrelevant.
+
+Other options are listed in the `README.md` and via the command-line option
+`-ce-help`.
+
+Counter-Examples also includes some GUI features, such as popup menu entries
+and a panel listing all generated counter-examples, with highlighting features.
+
+## Technical Notes
+
+- Since the logical theories in which WP's proof obligations are expressed are
+  undecidable, the logical model might be only partial. Namely, provers aim at
+  being correct (when they answer that the property is true, this is always the
+  case), and the price to pay is incompleteness (when they give a model that
+  might falsify the property, the model might be spurious).
+
+- The Counter-Examples plug-in itself is not always able to take into account
+  all the information provided by the prover, which make the counter-example
+  generated at the C level less conmplete.
+
+## Dependencies
+
+Counter-Examples depends mainly on results of the [WP](wp.html) plug-in.
+A few options depend on [Eva](eva.html).
+
+## Contact
+
+The plug-in is currently available under a proprietary licence.
+For any questions, remarks or suggestions, please contact
+[Virgile Prevosto](mailto:virgile.prevosto@cea.fr?subject=[Counter-Examples]).