This is the first pre-release of EiffelStudio 6.6. We added a new construct: the `check' statement which can be useful in void-safe mode which is like a check compound except that even if assertions are disabled and the assertion is false, you still get a check violation. We have redone our runtime multithreading and solved a few synchronization issues when threads have a very short lifespan and that you create many of them. Also we have prevented some deadlock in the runtime if a thread is killed outside the Eiffel environment. Added support for NaN, -Infinity and +Infinity in the runtime and the REAL_XX classes. Fixed ARRAY2 to really be void-safe in experimental mode. Various bug fixes in the runtime/compiler and environment.