CAS command question

[parisse]

Suppose you store your equation in a variable a:=(x+y=4), then you expect left(a) or right(a) not to error. The general rule is to eval arguments and leave the user quote himself if he requires, because once the system autoquote arguments, there is no way for the user to eval them.
According to the Prime online help, left and right return the left/right part of an equation or interval (Xcas documentation adds strings). Inequations are not cited. The only change I could make would be to error on inequations to prevent unadvertised users from being surprised, but I'm not sure everybody would like that (therefore I would do it for the Prime only).