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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 prune_before(self, timestamp):
 """Remove all resources with timestamp earlier than that given. Returns the number of entries removed. Will raise an excpetion if there are any entries without a timestamp. """

        n = 0
        pruned = []
        for r in self.resources:
            if (r.timestamp is None):
                raise Exception("Entry %s has no timestamp" % (r.uri))
            elif (r.timestamp >= timestamp):
                pruned.append(r)
            else:
                n += 1
        self.resources = pruned
        return(n)