Nostalgia

[parisse]

Yes, you can use Giac (not Xcas) inside sage, just install Giac/Xcas separately, the interface code is provided by sage. But this is hard way to use Giac, you need much more ressources (a full sage install), and the sage UI does not provide the same level of help. It might be of interest for maths people who have already code working in sage and want to accelerate it in some areas where Giac is much faster than sage (for example multivariate polynomials, Groebner basis, rational univariate representation...). Otherwise, it's easier to use Xcas or Qcas on the desktop or one of the existing Giac interfaces for programmers: python (giacpy module), javascript (webxcas), java (JNI)