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.

def decref_dependencies(self, term, refcounts):
        """
        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.
        """
        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