copy doc from one function to another
    use as a decorator eg::
        
        @_doc(file.tell)
        def tell(..):
            ...

def _doc(from_func):
    '''copy doc from one function to another
    use as a decorator eg::
        
        @_doc(file.tell)
        def tell(..):
            ...
    '''
    def decorator(to_func):
        to_func.__doc__ = from_func.__doc__
        return to_func
    return decorator