Newbie question: introducing symbols (variables) inside vectors and matrices

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Hi,

I would like to have linear algebra operations to be evaluated with symbols instead of numerically. For example, having the following matrix:

A = matrix(QQ,[
    [2,1,2,-6],
    [-1,2,1,7],
    [3,-1,-3,-1],
    [1,5,6,0],
    [2,2,1,1]
    ])

I would like to multiply for a vector with symbolic variables as follows:

t = 'real'
var('x1')
assume(x1,t)
var('x2')
assume(x2,t)
var('x3')
assume(x3,t)
var('x4')
assume(x4,t)
xx = vector(QQ,[x1,x2,x3,x4])
A * xx.transpose()

Unfortunately building the xx vector is unsuccessful, producing this error message:

TypeError: Cannot evaluate symbolic expression to a numeric value.

This does not work, so how can I use symbols in sage's linear algebra framework?

thank you very much, -Francesco