def dup_mirror(f, K):
    """
    Evaluate efficiently the composition ``f(-x)`` in ``K[x]``.

    Examples
    ========

    >>> R, x = ring('x', ZZ)

    >>> R.dup_mirror(x**3 + 2*x**2 - 4*x + 2)
    -x**3 + 2*x**2 + 4*x + 2

    """
    f = list(f)
    for i in range(len(f) - 2, -1, -2):
        f[i] = -f[i]
    return f
