One Sided Limit Error on the Prime

[parisse]

Xcas complex mode (translated by Use i in the Prime CAS settings) does not mean that complex result are forbidden. For example sqrt(-1) returns i in real mode. It is used if there is something ambiguous, as in the example you cite: factor(x^2+4), but also for sqrt(x^2).
I decided to do that after my experience with the HP49 CAS, where I found it was much too annoying to switch modes. The idea behind is that you should almost never have to work in complex mode (you can use cfactor or csolve to get complex factorization and roots in real mode). It's the same as for exact/approx mode : for example you don't want to get an error if you add a fraction and and approx number or if you call fsolve with exact data.
My experience with students using Xcas is that it's much easier that way (try to explain to a student on a hp49/50 that after each parametric plot he must switch complex mode off and exact mode on).