i know, it's not elegant, but `fetch` and `rewrite` are in the global scope. about mutability, unfortunately in javascript i got used to stateful functions, so even with a `doctest.options` object or a `doctest.setOptions` method, i would't be surprised by having successive calls affected by previous settings. d3 for example uses this concept extensively. but it's your choice, i don't have a strong opinion about this