code: def provStacks(self, offs, size):
          '''
          Returns a stream of provenance stacks at the given offset
          '''
          for _, iden in self.provseq.slice(offs, size):
              stack = self.getProvStack(iden)
              if stack is None:
                  continue
              yield (iden, stackf provStacks(self, offs, size):
          '''
          Returns a stream of provenance stacks at the given offset
          '''
          for _, iden in self.provseq.slice(offs, size):
              stack = self.getProvStack(iden)
              if stack is None:
                  continue
              yield (iden, stack)
docs: Returns a stream of provenance stacks at the given offset