This is different from a CAS. For example, if you ask it to do exp(100)+1-exp(100), it does not rearrange and cancel out the two exp(100)s. Instead, it does exp, addition, and subtraction, all with as many digits as you ask for.

Sure. I was speaking to the what, not how.

> that doesn’t round at all. It computes with constructive real numbers: every result is exact, and you can scroll any answer for as many correct digits as you want.

A CAS is a practical way to achieve this, where everything is stored symbolically, with no rounding, until final calculation. Unlimited precision was through Erable [1]. This was included with/as HP49 CAS, but was an add-on with HP48. Many HP48 are on the iOS app store. The one I tried about a decade ago had the add-on, and I see many still there (but not sure if they have the add-on).

The "S" in CAS is "system", so it's a bit open.

[1] https://en.wikipedia.org/wiki/Erable