Absolute value: sometimes CAS is very clever, sometimes not!

[parisse]

It works in Xcas for PC, not in Xcas for Firefox, not on the Prime. If my recollection is correct, this is because on these architectures where we have less computation power, essentially nothing is done for expression with more than one variable, because that would hang for some more complex expressions.
If there are important uses like you describe, I can probably add some code for simple multivariate expressions, where you can deduce the sign without advanced math algorithms. You can contact me directly in French by email or on the Xcas forum to discuss use of Xcas and calculator ports for science teaching.

To toml_12953: speak is easy. There is no bug here, it's just that implementation may be hard if you want to be able to run in all cases.