def dual_elt(AS, zmag):
    '''Find the trace dual of a given elt zmag in the function field of an Artin-Schreier cover AS.'''
    p = AS.characteristic
    n = AS.height
    group_elts = [(i, j) for i in range(p) for j in range(p)]
    variable_names = 'x, y'
    for i in range(n):
        variable_names += ', z' + str(i)
    Rxyz = PolynomialRing(F, n+2, variable_names)
    x, y = Rxyz.gens()[:2]
    z = Rxyz.gens()[2:]
    RxyzQ = FractionField(Rxyz)
    M = matrix(RxyzQ, p^n, p^n)
    for i in range(p^n):
        for j in range(p^n):
            M[i, j] = (zmag.group_action(group_elts[i])*zmag.group_action(group_elts[j])).trace2()
    main_det = M.determinant()
    zvee = as_function(AS, 0)
    for i in range(p^n):
        Mprim = matrix(RxyzQ, M)
        Mprim[:, i] = vector([(j == 0) for j in range(p^2)])
        fi = Mprim.determinant()/main_det
        zvee += fi*zmag.group_action(group_elts[i])
    return zvee