If you are using a library with this kind of number representation, computing any rational number with a repeating decimal representation will use up all your memory. 1/3=0.33333… It will keep allocating memory to store infinite copies of the digit 3. (In practice it stores it using binary representation but you get the idea.)
For the Rust crate, there is already an arbitrary limit (defaults to 100 digits) for "unbounded operations" like square_root, inverting, division. That's a compile time constant. And there's a Context object for runtime-configuration you can set with a precision (stop after `prec` digits).
But for addition, the idea is to give the complete number if you do `a + b`, otherwise you could use the context to keep the numbers within your `ctx.add(a, b)`. But after the discussions here, maybe this is too unsafe... and it should use the default precision (or a slightly larger one) in the name of safety? With a compile time flag to disable it? hmm...
