'Return a URL only if accessible by the user, otherwise False
- used for Navigation Items
@param c: the controller
@param f: the function
@param p: the permission (defaults to READ)
@param t: the tablename (defaults to <c>_<f>)
@param a: the application name
@param args: the URL arguments
@param vars: the URL variables
@param anchor: the anchor (#) of the URL
@param extension: the request format extension
@param env: the environment'

def accessible_url(self, c=None, f=None, p=None, t=None, a=None, args=[], vars={}, anchor='', extension=None, env=None):

    if (c != 'static'):
        settings = current.deployment_settings
        if (not settings.has_module(c)):
            return False
    if (t is None):
        t = ('%s_%s' % (c, f))
        table = current.s3db.table(t)
        if (not table):
            t = None
    if (not p):
        p = 'read'
    permitted = self.has_permission(p, c=c, f=f, t=t)
    if permitted:
        return URL(a=a, c=c, f=f, args=args, vars=vars, anchor=anchor, extension=extension, env=env)
    else:
        return False
