From 1d69539b9b82b290a510bb3046c49f300f762c0d Mon Sep 17 00:00:00 2001
From: John Lancaster <32917998+jsl12@users.noreply.github.com>
Date: Sat, 10 Oct 2026 11:04:53 -0500
Subject: [PATCH] subgraph isolation

---
 src/daglib/dag.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/daglib/dag.py b/src/daglib/dag.py
index 4a9747e..2933f2b 100644
--- a/src/daglib/dag.py
+++ b/src/daglib/dag.py
@@ -23,7 +23,9 @@ class DAG[T](MutableMapping[T, DAGSetView[T]]):
     """
 
     _succ: dictset[T] = field(default_factory=lambda: defaultdict(set))
+    """Mapping of nodes to the set of nodes that succeed them in the DAG."""
     _pred: dictset[T] = field(default_factory=lambda: defaultdict(set))
+    """Mapping of nodes to the set of nodes that precede them in the DAG."""
 
     on_add: Callable[[T, T], None] | None = None
     on_remove: Callable[[T, T], None] | None = None
@@ -119,10 +121,11 @@ class DAG[T](MutableMapping[T, DAGSetView[T]]):
         self._pred.pop(u, None)
 
     def subgraph(self, sub: T | Iterable[T]) -> Self:
+        """Return an independent graph induced by the successor closure of sub."""
         closure = transitive_closure(self, sub)
         subgraph = type(self)(
-            _succ=defaultdict(set, {k: self._succ[k] for k in set(self._succ.keys()) & closure}),
-            _pred=defaultdict(set, {k: self._pred[k] for k in set(self._pred.keys()) & closure}),
+            _succ=defaultdict(set, {node: self._succ[node] & closure for node in self._succ.keys() & closure}),
+            _pred=defaultdict(set, {node: self._pred[node] & closure for node in self._pred.keys() & closure}),
         )
         subgraph.on_add = self.on_add
         subgraph.on_remove = self.on_remove