def dup_lshift(f, n, K):
    """
    Efficiently multiply ``f`` by ``x**n`` in ``K[x]``.

    Examples
    ========

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

    >>> R.dup_lshift(x**2 + 1, 2)
    x**4 + x**2

    """
    if not f:
        return f
    else:
        return f + [K.zero] * n
