Transform a matrix into a quadratic form : a2q

a2q takes two arguments : the symetric matrix A of a quadratic form q and a vector of variable names of the same size.
a2q returns the symbolic expression of quadratic form q.
Input :

a2q([[0,1],[1,0]],[x,y])

Output :

2*x*y

Input :

a2q([[1,2],[2,4]],[x,y])

Output :

x^2+4*x*y+4*y^2