fsolve() crash with given Interval

[parisse]

I have never seen a CAS where you must make an explicit call to replace a variable by the assigned value. Because you will want to store equations, expressions, functions and you don't want to enter something like eval(f)(eval(x0),eval(y0)).
But there is a mechanism in Xcas that allows keeping the symbolic value if evaluation is exact and replacing by the numeric value if evaluation is approx (evalf), it's a slider. Try assume(a=[0,-5,5,0.1]), then factor(a^2-1) and a^2-1.0. I did not try it on the Prime, I guess it should work, but you will not be able to change the numeric value of the slider interactively like in Xcas.