|  | @@ -56,10 +56,14 @@ struct ExpressionRef {
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				|  |  |    // must work for T = Jet<ExpressionRef>.
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				|  |  |    ExpressionRef(double compile_time_constant);
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				|  |  |  
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				|  |  | -  // By adding this constructor (which always throws an error) we can detect
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				|  |  | -  // invalid usage of ExpressionRef. ExpressionRef can only be created from
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				|  |  | -  // constexpr doubles.
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				|  |  | -  ExpressionRef(double& test);
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				|  |  | +  // By adding this deleted constructor we can detect invalid usage of
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				|  |  | +  // ExpressionRef. ExpressionRef must only be created from constexpr doubles.
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				|  |  | +  //
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				|  |  | +  // If you get a compile error here, you have probably written something like:
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				|  |  | +  //   T x = local_variable_;
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				|  |  | +  // Change this into:
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				|  |  | +  //   T x = CERES_LOCAL_VARIABLE(local_variable_);
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				|  |  | +  ExpressionRef(double&) = delete;
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				|  |  |  
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				|  |  |    // Create an ASSIGNMENT expression from other to this.
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				|  |  |    //
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