diff --git a/src/daglib/set.py b/src/daglib/set.py
index 954c931..3c13180 100644
--- a/src/daglib/set.py
+++ b/src/daglib/set.py
@@ -50,9 +50,10 @@ class DAGSet(MutableSet[T]):
             self.on_add(v)
 
     def discard(self, v: T) -> None:
-        self._data.discard(v)
-        if self.on_remove:
-            self.on_remove(v)
+        if v in self._data:
+            self._data.discard(v)
+            if self.on_remove:
+                self.on_remove(v)
 
     # --- in-place operator support ---
 
@@ -69,22 +70,38 @@ class DAGSet(MutableSet[T]):
                 self |= {other}
         return self
 
-    # def __isub__(self, other: Iterable[T]) -> DAGSet[T]:
-    #     # a -= b => remove those in other
-    #     match other:
-    #         case set():
-    #             self -= other
-    #         case _:
-    #             self -= set(other)
-    #     return self
+    def __isub__(self, other: Iterable[T]) -> DAGSet[T]:
+        # a -= b => remove those in other
+        match other:
+            case str():
+                other = {other}
+            case Iterable():
+                other = set(other)
+            case _:
+                other = {other}
+        return super().__isub__(other)
 
-    # def __iand__(self, other: Iterable[T]) -> DAGSet[T]:
-    #     # a &= b => keep only those also in other
-    #     return self
+    def __iand__(self, other: Iterable[T]) -> DAGSet[T]:
+        # a &= b => keep only those also in other
+        match other:
+            case str():
+                other = {other}
+            case Iterable():
+                other = set(other)
+            case _:
+                other = {other}
+        return super().__iand__(other)
 
-    # def __ixor__(self, other: Iterable[T]) -> DAGSet[T]:
-    #     # a ^= b => symmetric difference update
-    #     return self
+    def __ixor__(self, other: Iterable[T]) -> DAGSet[T]:
+        # a ^= b => symmetric difference update
+        match other:
+            case str():
+                other = {other}
+            case Iterable():
+                other = set(other)
+            case _:
+                other = {other}
+        return super().__ixor__(other)
 
     # --- nice repr for debugging ---