'Return the target URL for this item if accessible by the
current user, otherwise False
@param extension: override the format extension
@param kwargs: override URL query vars'

def accessible_url(self, extension=None, **kwargs):

    aURL = current.auth.permission.accessible_url
    if (not self.link):
        return None
    args = self.args
    if self.vars:
        link_vars = Storage(self.vars)
        link_vars.update(kwargs)
    else:
        link_vars = Storage(kwargs)
    if (extension is None):
        extension = self.extension
    a = self.get('application')
    if (a is None):
        a = current.request.application
    c = self.get('controller')
    if (c is None):
        c = 'default'
    f = self.get('function')
    if (f is None):
        f = 'index'
    (f, args) = self.__format(f, args, extension)
    return aURL(c=c, f=f, p=self.p, a=a, t=self.tablename, args=args, vars=link_vars)
