code: def prune_before(self, 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)
docs: 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.