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Esterel is a unique programming language designed for the development of safety-critical systems. It is based on the synchronous programming model, which allows for the easy development of highly concurrent and real-time systems. The language was developed in the 1980s by a team at the French Institute for Research in Computer Science and Control (INRIA), led by Professor Gerard Berry. Esterel has been used in a wide range of applications, including the development of the Airbus A380 aircraft control systems, the French Ariane 5 rocket control system, and the Japanese Superconducting Tokamak Fusion Reactor. The synchronous programming model used by Esterel is based on the concept of "signals" which are variables that can take on a finite set of values. These values are typically Boolean (true/false) or integer (0, 1, 2, etc.). A signal can only change value at specific points in time, called "events". Events can be either "external" (triggered by an external input, such as a button press) or "internal" (triggered by the program itself). Esterel programs are constructed from a set of "modules" which are connected together to form a directed graph. Each module has a set of input and output ports, and a set of "signals" which are connected to the ports. Modules can be connected together in any order, and the order in which they are executed is determined by the "signal flow" through the graph. The Esterel compiler will automatically generate code that will execute the modules in the correct order, based on the signal flow. This allows for the easy development of highly concurrent systems, as the programmer does not need to worry about the order in which the modules will be executed. The use of the synchronous programming model also allows for the development of real-time systems, as the timing of events can be precisely controlled. Esterel programs can be compiled to run on a wide range of platforms, including embedded systems. The Esterel language has been very successful in the development of safety-critical systems, due to its ability to easily develop concurrent and real-time systems. It has been used in a wide variety of applications, including aircraft control systems, rocket control systems, and fusion reactor control systems.