diff --git a/src/rcore.rs b/src/rcore.rs
index a8581f7..9bd7d5e 100644
--- a/src/rcore.rs
+++ b/src/rcore.rs
@@ -76,7 +76,7 @@ pub trait Factory<'a, Ctx: Context<'a>>: 'a + Send + Sync + Clone {
         addresses: &mut Addresses,
     ) -> ParseResult<'a, Ctx, Self>;
     /// Called by finite stream parsers if there's any data left.
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError;
+    fn extend(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Result<Self::Mtbl, Self::ParseError>;
 }
 
 pub type Mtbl<'a, Ctx, F> = <F as Factory<'a, Ctx>>::Mtbl;
diff --git a/src/rcore/addresses.rs b/src/rcore/addresses.rs
index 33ca875..31aac60 100644
--- a/src/rcore/addresses.rs
+++ b/src/rcore/addresses.rs
@@ -51,7 +51,7 @@ fn _parse_slice<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>>(
     if tail.is_empty() {
         Ok(mentionable)
     } else {
-        Err(factory.unexpected_tail(mentionable, tail))
+        factory.extend(mentionable, tail)
     }
 }
 
diff --git a/src/rstd/atomic.rs b/src/rstd/atomic.rs
index 15f15fb..673e1c8 100644
--- a/src/rstd/atomic.rs
+++ b/src/rstd/atomic.rs
@@ -20,8 +20,8 @@ pub trait Atomic: 'static + Send + Sync + Send + Clone + Serializable {
     type AParseError: Error;
     /// Static equivalent of [`Factory::deserialize`].
     fn a_deserialize(deserializer: &mut dyn Deserializer) -> Result<Self, Self::AParseError>;
-    /// Static equivalent of [`Factory::unexpected_tail`].
-    fn a_unexpected_tail(self, tail: &[u8]) -> Self::AParseError;
+    /// Static equivalent of [`Factory::extend`].
+    fn a_extend(self, tail: &[u8]) -> Result<Self, Self::AParseError>;
 }
 
 fn _parse_slice<A: Atomic>(slice: &[u8]) -> Result<A, A::AParseError> {
@@ -31,7 +31,7 @@ fn _parse_slice<A: Atomic>(slice: &[u8]) -> Result<A, A::AParseError> {
     if tail.is_empty() {
         Ok(atomic)
     } else {
-        Err(A::a_unexpected_tail(atomic, tail))
+        A::a_extend(atomic, tail)
     }
 }
 
diff --git a/src/rstd/atomic/atomic_object.rs b/src/rstd/atomic/atomic_object.rs
index bb94d52..9b008e9 100644
--- a/src/rstd/atomic/atomic_object.rs
+++ b/src/rstd/atomic/atomic_object.rs
@@ -81,8 +81,8 @@ impl<'a, Ctx: Context<'a>, A: Atomic> Factory<'a, Ctx> for AtomicFactory<A> {
         Ok(A::a_deserialize(deserializer)?.into())
     }
 
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        A::a_unexpected_tail(mentionable.atomic, tail)
+    fn extend(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Result<Self::Mtbl, Self::ParseError> {
+        Ok(A::a_extend(mentionable.atomic, tail)?.into())
     }
 }
 
diff --git a/src/rstd/atomic/au64.rs b/src/rstd/atomic/au64.rs
index 2c81293..52b0359 100644
--- a/src/rstd/atomic/au64.rs
+++ b/src/rstd/atomic/au64.rs
@@ -40,8 +40,8 @@ impl Atomic for u64 {
         Ok(u64::from_le_bytes(deserializer.read_n_const::<8>()?))
     }
 
-    fn a_unexpected_tail(self, tail: &[u8]) -> Self::AParseError {
-        IntParseError::ExtraData(tail.len())
+    fn a_extend(self, tail: &[u8]) -> Result<Self, Self::AParseError> {
+        Err(IntParseError::ExtraData(tail.len()))
     }
 }
 
diff --git a/src/rstd/atomic/boolean.rs b/src/rstd/atomic/boolean.rs
index 740bd81..b491309 100644
--- a/src/rstd/atomic/boolean.rs
+++ b/src/rstd/atomic/boolean.rs
@@ -52,8 +52,8 @@ impl Atomic for bool {
         }
     }
 
-    fn a_unexpected_tail(self, tail: &[u8]) -> Self::AParseError {
-        BooleanParseError::ExtraData(tail.len())
+    fn a_extend(self, tail: &[u8]) -> Result<Self, Self::AParseError> {
+        Err(BooleanParseError::ExtraData(tail.len()))
     }
 }
 
diff --git a/src/rstd/atomic/plain.rs b/src/rstd/atomic/plain.rs
index 5da29f5..db71d16 100644
--- a/src/rstd/atomic/plain.rs
+++ b/src/rstd/atomic/plain.rs
@@ -35,11 +35,9 @@ impl Atomic for Plain {
         Ok(Plain::from_slice(deserializer.read_all()))
     }
 
-    fn a_unexpected_tail(self, tail: &[u8]) -> Self::AParseError {
-        panic!(
-            "Plain must use read_all, therefore there must not be any extra tail (received {} bytes).",
-            tail.len()
-        )
+    fn a_extend(mut self, tail: &[u8]) -> Result<Self, Self::AParseError> {
+        self.data.extend_from_slice(tail);
+        Ok(self)
     }
 }
 
diff --git a/src/rstd/collections/avl.rs b/src/rstd/collections/avl.rs
index 8ded131..24c0a37 100644
--- a/src/rstd/collections/avl.rs
+++ b/src/rstd/collections/avl.rs
@@ -149,8 +149,16 @@ impl<'a, Ctx: Context<'a>, F: Factory<'a, Ctx>> Factory<'a, Ctx> for AvlNodeFact
         Ok(AvlNode { l, r, key })
     }
 
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        AvlError::Key(self.0.unexpected_tail(mentionable.key, tail))
+    fn extend(
+        &self,
+        mut mentionable: Self::Mtbl,
+        tail: &[u8],
+    ) -> Result<Self::Mtbl, Self::ParseError> {
+        mentionable.key = self
+            .0
+            .extend(mentionable.key, tail)
+            .map_err(AvlError::Key)?;
+        Ok(mentionable)
     }
 }
 
@@ -177,7 +185,12 @@ impl<'a, Ctx: Context<'a>, F: Factory<'a, Ctx>> Factory<'a, Ctx> for AvlTreeFact
         Ok(AvlTree { node, height })
     }
 
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        u64::a_unexpected_tail(mentionable.height, tail).into()
+    fn extend(
+        &self,
+        mut mentionable: Self::Mtbl,
+        tail: &[u8],
+    ) -> Result<Self::Mtbl, Self::ParseError> {
+        mentionable.height = u64::a_extend(mentionable.height, tail)?;
+        Ok(mentionable)
     }
 }
diff --git a/src/rstd/collections/stack.rs b/src/rstd/collections/stack.rs
index 45bb292..fd4ff48 100644
--- a/src/rstd/collections/stack.rs
+++ b/src/rstd/collections/stack.rs
@@ -102,11 +102,16 @@ impl<'a, Ctx: Context<'a>, F: Factory<'a, Ctx>> Factory<'a, Ctx> for StackNodeFa
         Ok(StackNode { rest, element })
     }
 
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        StackParseError::Element(
-            self.element_factory
-                .unexpected_tail(mentionable.element, tail),
-        )
+    fn extend(
+        &self,
+        mut mentionable: Self::Mtbl,
+        tail: &[u8],
+    ) -> Result<Self::Mtbl, Self::ParseError> {
+        mentionable.element = self
+            .element_factory
+            .extend(mentionable.element, tail)
+            .map_err(StackParseError::Element)?;
+        Ok(mentionable)
     }
 }
 
diff --git a/src/rstd/inlining/static_pair.rs b/src/rstd/inlining/static_pair.rs
index ecfd66d..939fdb7 100644
--- a/src/rstd/inlining/static_pair.rs
+++ b/src/rstd/inlining/static_pair.rs
@@ -151,10 +151,13 @@ impl<'a, Ctx: Context<'a>, SP: StaticPair<'a, Ctx>> Factory<'a, Ctx>
         })
     }
 
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
+    fn extend(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Result<Self::Mtbl, Self::ParseError> {
         let (_, fb) = SP::factories(&self.factory_data);
-        let (_, b) = mentionable.pair.into_elements();
-        SP::from_error_b(&self.factory_data, fb.unexpected_tail(b, tail))
+        let (a, b) = mentionable.pair.into_elements();
+        match fb.extend(b, tail) {
+            Ok(b) => Ok(SP::from_parsed(&self.factory_data, a, b).into()),
+            Err(e) => Err(SP::from_error_b(&self.factory_data, e)),
+        }
     }
 }
 
diff --git a/src/rstd/nullable.rs b/src/rstd/nullable.rs
index 6011e6b..e075b34 100644
--- a/src/rstd/nullable.rs
+++ b/src/rstd/nullable.rs
@@ -70,8 +70,12 @@ impl<'a, Ctx: Context<'a>, F: Factory<'a, Ctx>> Factory<'a, Ctx> for NullableFac
         })
     }
 
-    fn unexpected_tail(&self, _mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        PointParseError::WrongLength(HASH_SIZE + tail.len())
+    fn extend(
+        &self,
+        _mentionable: Self::Mtbl,
+        tail: &[u8],
+    ) -> Result<Self::Mtbl, Self::ParseError> {
+        Err(PointParseError::WrongLength(HASH_SIZE + tail.len()))
     }
 }
 
diff --git a/src/rstd/point.rs b/src/rstd/point.rs
index cd1535a..bd771e9 100644
--- a/src/rstd/point.rs
+++ b/src/rstd/point.rs
@@ -77,7 +77,11 @@ impl<'a, Ctx: Context<'a>, F: Factory<'a, Ctx>> Factory<'a, Ctx> for PointFactor
         Ok(addresses.next_point(deserializer, resolver, self.factory.clone())?)
     }
 
-    fn unexpected_tail(&self, _mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        PointParseError::WrongLength(HASH_SIZE + tail.len())
+    fn extend(
+        &self,
+        _mentionable: Self::Mtbl,
+        tail: &[u8],
+    ) -> Result<Self::Mtbl, Self::ParseError> {
+        Err(PointParseError::WrongLength(HASH_SIZE + tail.len()))
     }
 }
diff --git a/src/rstd/typeless.rs b/src/rstd/typeless.rs
index c113a01..45377b5 100644
--- a/src/rstd/typeless.rs
+++ b/src/rstd/typeless.rs
@@ -30,7 +30,11 @@ type TdeBox<'a, Ctx> = Box<dyn Tde<'a, Ctx>>;
 trait Tut<'a, Ctx: Context<'a>>: 'a + Send + Sync {
     fn clone_box(&self) -> TutBox<'a, Ctx>;
 
-    fn ut(&self, mentionable: TypelessMentionable<'a, Ctx>, tail: &[u8]) -> TypelessError<'a>;
+    fn xt(
+        &self,
+        mentionable: TypelessMentionable<'a, Ctx>,
+        tail: &[u8],
+    ) -> Result<TypelessMentionable<'a, Ctx>, TypelessError<'a>>;
 }
 
 type TutBox<'a, Ctx> = Box<dyn Tut<'a, Ctx>>;
@@ -38,7 +42,7 @@ type TutBox<'a, Ctx> = Box<dyn Tut<'a, Ctx>>;
 /// See [`Point::typeless`]/[`TypelessMentionable`].
 pub struct TypelessFactory<'a, Ctx: Context<'a>> {
     t_deserialize: TdeBox<'a, Ctx>,
-    t_unexpected_tail: TutBox<'a, Ctx>,
+    t_extend: TutBox<'a, Ctx>,
 }
 
 impl<'a, Ctx: Context<'a>> Serializable for TypelessMentionable<'a, Ctx> {
@@ -69,7 +73,7 @@ impl<'a, Ctx: Context<'a>> Clone for TypelessFactory<'a, Ctx> {
     fn clone(&self) -> Self {
         Self {
             t_deserialize: self.t_deserialize.clone_box(),
-            t_unexpected_tail: self.t_unexpected_tail.clone_box(),
+            t_extend: self.t_extend.clone_box(),
         }
     }
 }
@@ -103,8 +107,8 @@ impl<'a, Ctx: Context<'a>> Factory<'a, Ctx> for TypelessFactory<'a, Ctx> {
         }
     }
 
-    fn unexpected_tail(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Self::ParseError {
-        self.t_unexpected_tail.ut(mentionable, tail)
+    fn extend(&self, mentionable: Self::Mtbl, tail: &[u8]) -> Result<Self::Mtbl, Self::ParseError> {
+        self.t_extend.xt(mentionable, tail)
     }
 }
 
@@ -157,14 +161,21 @@ where
         Box::new(self.clone())
     }
 
-    fn ut(&self, mentionable: TypelessMentionable<'a, Ctx>, tail: &[u8]) -> TypelessError<'a> {
-        TypelessError::from_typed(self.unexpected_tail(
+    fn xt(
+        &self,
+        mentionable: TypelessMentionable<'a, Ctx>,
+        tail: &[u8],
+    ) -> Result<TypelessMentionable<'a, Ctx>, TypelessError<'a>> {
+        self.extend(
             match mentionable.cast(self.clone()) {
                 Ok(m) => m,
-                Err(e) => return TypelessError::from_typed(e),
+                Err(e) => return Err(TypelessError::from_typed(e)),
             },
             tail,
-        ))
+        )
+        .map_err(TypelessError::from_typed)
+        .map(Rc::new)
+        .map(TypelessMentionable::from_typed)
     }
 }
 
@@ -175,7 +186,7 @@ where
     pub fn from_typed<F: Factory<'a, Ctx>>(factory: F) -> Self {
         TypelessFactory {
             t_deserialize: Box::new(factory.clone()),
-            t_unexpected_tail: Box::new(factory),
+            t_extend: Box::new(factory),
         }
     }
 }