'Unregister files handled by these functions.
:param set removed_functions:
Set of pairs (2-tuples) representing ``(condition_function,
function)`` function pairs previously passed as an arguments to
``.register_missing()`` method.'

def unregister_missing(self, removed_functions):

    with self.lock:
        for (key, functions) in list(self.missing.items()):
            functions -= removed_functions
            if (not functions):
                self.missing.pop(key)
