@bluescarni yes, Arb is for certified accuracy. But Fredrik has implemented some very fast algorithms there and I think it has better support for special functions than mpfr. For CAS, you really want to know the accuracy of the final answer. However, I think it will be valuable to have various ways to evaluate an expression, using double, quadmath, mpfr, Arb, .... I think the visitor pattern should allow one to easily hook his own favorite library.