def missing_branch_arcs(self):
        """Return arcs that weren't executed from branch lines.

        Returns {l1:[l2a,l2b,...], ...}

        """
        missing = self.arcs_missing()
        branch_lines = set(self.branch_lines())
        mba = {}
        for l1, l2 in missing:
            if l1 in branch_lines:
                if l1 not in mba:
                    mba[l1] = []
                mba[l1].append(l2)
        return mba
Return arcs that weren't executed from branch lines.

        Returns {l1:[l2a,l2b,...], ...}