These help constrain what are valid equations at compile time thereby preventing invalid equations from causing runtime crashes due to missing methods and members (this does not validate anythinng about the physics of an equation, only that the form of the equation is right). Note that this is done at compile time rather than with runtime polymorphism to avoid both indirection cost but more imporantly to allow the compiler to more easily inline functions and preform other optimizations that indirection can hide.
Emily Boudreaux
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