def __rall_power(parent_diam,e=1.5):
    """
    Returns the diameter of a child segment of a branch 
    according to Rall's Power Law as described
    in Van Ooyen et al 2010. Assumes child branches
    will be of equal diameter.
    """

    child_diam=parent_diam/(2**(1/e))
    return child_diam