>>> import callback
>>> print(callback.foo.__doc__)
r = foo(fun,[fun_extra_args])

Wrapper for ``foo``.

Parameters
----------
fun : call-back function

Other Parameters
----------------
fun_extra_args : input tuple, optional
    Default: ()

Returns
-------
r : float

Notes
-----
Call-back functions::

  def fun(i): return r
  Required arguments:
    i : input int
  Return objects:
    r : float

>>> def f(i): return i*i
... 
>>> print(callback.foo(f))
110.0
>>> print(callback.foo(lambda i:1))
11.0