def missing_branch_arcs(self) -> dict[TLineNo, list[TLineNo]]:
        """Return arcs that weren't executed from branch lines.

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

        """
        missing = self.arcs_missing()
        branch_lines = set(self._branch_lines())
        mba = collections.defaultdict(list)
        for l1, l2 in missing:
            assert l1 != l2, f"In {self.filename}, didn't expect {l1} == {l2}"
            if l1 in branch_lines:
                mba[l1].append(l2)
        return mba
Return arcs that weren't executed from branch lines.

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

        
coverage/results.py