def compute_deriv(poly):
    """
    Computes and returns the derivative of a polynomial function. If the
    derivative is 0, returns (0.0,).

    Example:
    # >>> poly = (-13.39, 0.0, 17.5, 3.0, 1.0)    # x^4 + 3x^3 + 17.5x^2 - 13.39
    # >>> print compute_deriv(poly)        # 4x^3 + 9x^2 + 35^x
    (0.0, 35.0, 9.0, 4.0)

    poly: tuple of numbers, length > 0
    returns: tuple of numbers
    """
    i = 1
    deriv = ()
    for x in poly[1:]:
        y = x*i
        i += 1
        deriv += (y,)
    return deriv