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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def append_function(self, function):
 """Append the function to the list of functions to be called. If the function is already a callable, use it. If it's a type str try to interpret it as [module]:?<callable>, load the module if there is one and retrieve the callable. Argument: function (str or callable):
 function to call on input. """

        if not hasattr(function, '__call__'):
            function = get_function(function)
            if not hasattr(function, '__call__'):
                raise ValueError("function is expected to be callable")
        self._functions.append(function)
        log.debug("registered %s", function.__name__)