[Maxima] changing an equation into an allocation

[Sara Pashmin]

Hello,

I try to eliminate the right side of the following equation, which I
got from algsys-function:

a = x+y

but rhs(a) gives the answer: "0"

For "a : x+y " rhs(a) gives the answer "x+y".

Now it is my problem to change an equation into an allocation. I mean
to change the equation "a=x+y" to "a: x+y" for eliminating the right
side of the equation.

I would be happy for any help.

Thanks and regards,

Sara