code: def decref_dependencies(self, term, refcounts):
          
          garbage = set()
          # Edges are tuple of (from, to).
          for parent, _ in self.graph.in_edges([term]):
              refcounts[parent] -= 1
              # No one else depends on this term. Remove it from the
              # workspace to conserve memory.
              if refcounts[parent] == 0:
                  garbage.add(parent)
          return garbage
docs: Decrement in-edges for ``term`` after computation.
  
          Parameters
          ----------
          term : zipline.pipeline.Term
              The term whose parents should be decref'ed.
          refcounts : dict[Term -> int]
              Dictionary of refcounts.
  
          Return
          ------
          garbage : set[Term]
              Terms whose refcounts hit zero after decrefing.