  Among formal methods, the deductive verification approach allows establishing
the strongest possible formal guarantees on critical software. The downside is
the cost in terms of human effort required to design adequate formal
specifications and to successfully discharge the required proof obligations. To
popularize deductive verification in an industrial software development
environment, it is essential to provide means to progressively transition from
simple and automated approaches to deductive verification. The SPARK
environment, for development of critical software written in Ada, goes towards
this goal by providing automated tools for formally proving that some code
fulfills the requirements expressed in Ada contracts. In a program verifier
that makes use of automatic provers to discharge the proof obligations, a need
for some additional user interaction with proof tasks shows up: either to help
analyzing the reason of a proof failure or, ultimately, to discharge the
verification conditions that are out-of-reach of state-of-the-art automatic
provers. Adding interactive proof features in SPARK appears to be complicated
by the fact that the proof toolchain makes use of the independent, intermediate
verification tool Why3, which is generic enough to accept multiple front-ends
for different input languages. This paper reports on our approach to extend
Why3 with interactive proof features and also with a generic client-server
infrastructure allowing integration of proof interaction into an external,
front-end graphical user interface such as the one of SPARK.
