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Minor Description
I'm writing some code which outputs fractional measurements meant for human consumption. I need a constructor for Fraction which allows you to restrict the denominators to a finite set. This is necessary due to the fact that real-world tools are only available with certain fractions. For example, it's next to impossible to find a ruler with 1/7 inch marked 
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I'm attaching a patch which implements the functionality. I've attempted to mimic the style of the existing code as much as possible. One caveat: I don't speak French so the french error message is a computer-generated translation and probably very poor.