diff --git a/sleepy/core/__init__.py b/sleepy/core/__init__.py
index faa7bdc..0ba7111 100644
--- a/sleepy/core/__init__.py
+++ b/sleepy/core/__init__.py
@@ -1,2 +1,3 @@
 from .error import SleepyError
+from .meta_table import UID, Identifiable, MetaTable
 from .source_location import SourceLocation
diff --git a/sleepy/core/meta_table.py b/sleepy/core/meta_table.py
new file mode 100644
index 0000000..e1b3ba5
--- /dev/null
+++ b/sleepy/core/meta_table.py
@@ -0,0 +1,30 @@
+from abc import ABC, abstractproperty
+from typing import Generic, NewType, TypeVar
+
+UID = NewType("UID", int)
+
+T = TypeVar("T")
+
+
+class Identifiable(ABC):
+    @abstractproperty
+    def uid(self) -> UID:
+        raise NotImplementedError
+
+
+class MetaTable(Generic[T]):
+    def __init__(self) -> None:
+        self.entries: dict[UID, T] = {}
+
+    def __getitem__(self, key: Identifiable) -> T:
+        try:
+            return self.entries[key.uid]
+        except KeyError as e:
+            e.add_note(f"key: {key!r}")
+            raise
+
+    def __setitem__(self, key: Identifiable, value: T) -> None:
+        if key.uid in self.entries:
+            message = f"can't assign {value}, to key with id {key.uid}: {key!r}"
+            raise KeyError(message)
+        self.entries[key.uid] = value
diff --git a/sleepy/program/__init__.py b/sleepy/program/__init__.py
index de1ca2e..9e6f6d7 100644
--- a/sleepy/program/__init__.py
+++ b/sleepy/program/__init__.py
@@ -21,7 +21,6 @@
     Program,
     ProgramNode,
     Symbol,
-    SymbolId,
 )
 from .unit import ProgramUnit
 from .visitor import Visitor
diff --git a/sleepy/program/bindings.py b/sleepy/program/bindings.py
index cbb5be0..7bc03bb 100644
--- a/sleepy/program/bindings.py
+++ b/sleepy/program/bindings.py
@@ -1,6 +1,8 @@
 from abc import ABC, abstractmethod
 from typing import override
 
+from sleepy.core import MetaTable
+
 from .representation import Expression, Symbol
 
 
@@ -17,14 +19,14 @@ def resolve(self, symbol: Symbol) -> Expression:
 class BasicBindings(Bindings):
     def __init__(
         self,
-        expressions: dict[int, Expression] | None = None,
+        expressions: MetaTable[Expression] | None = None,
     ) -> None:
-        self.expressions = expressions or {}
+        self.expressions = expressions or MetaTable()
 
     @override
     def bind(self, symbol: Symbol, expression: Expression) -> None:
-        self.expressions[symbol.uid] = expression
+        self.expressions[symbol] = expression
 
     @override
     def resolve(self, symbol: Symbol) -> Expression:
-        return self.expressions[symbol.uid]
+        return self.expressions[symbol]
diff --git a/sleepy/program/builtin.py b/sleepy/program/builtin.py
index 54f6965..4dd4636 100644
--- a/sleepy/program/builtin.py
+++ b/sleepy/program/builtin.py
@@ -98,7 +98,5 @@ def __init__(self) -> None:
         self.namespace = LocalNamespace()
         self.bindings = BasicBindings()
         for intrinsic in intrinsics:
-            self.bindings.bind(
-                self.namespace.define(intrinsic.name),
-                intrinsic,
-            )
+            defined = self.namespace.define(intrinsic.name)
+            self.bindings.bind(defined, intrinsic)
diff --git a/sleepy/program/exception.py b/sleepy/program/exception.py
new file mode 100644
index 0000000..cfd7403
--- /dev/null
+++ b/sleepy/program/exception.py
@@ -0,0 +1,19 @@
+from sleepy.core import SleepyError
+
+
+class SemanticError(SleepyError):
+    pass
+
+
+class NamespaceError(SleepyError):
+    pass
+
+
+class DefinitionError(NamespaceError):
+    def __init__(self, name: str, reason: str) -> None:
+        super().__init__(f"failed to define a name {name}: {reason}")
+
+
+class NameNotFoundError(NamespaceError):
+    def __init__(self, name: str) -> None:
+        super().__init__(f"failed to resolve a name {name}: not found")
diff --git a/sleepy/program/namespace.py b/sleepy/program/namespace.py
index 3ebb9b8..50d002c 100644
--- a/sleepy/program/namespace.py
+++ b/sleepy/program/namespace.py
@@ -1,29 +1,10 @@
 from abc import ABC, abstractmethod
 from typing import override
 
-from sleepy.core import SleepyError
-
+from .exception import DefinitionError, NameNotFoundError
 from .representation import Symbol
 
 
-class NamespaceError(SleepyError):
-    pass
-
-
-class DefinitionError(NamespaceError):
-    def __init__(self, name: str, reason: str) -> None:
-        self.name = name
-        super().__init__(f"Failed to define a name {name}: {reason}")
-
-
-class NameNotFoundError(NamespaceError):
-    def __init__(self, name: str) -> None:
-        self.name = name
-        super().__init__(
-            f"Failed to resolve a name {name}: not found",
-        )
-
-
 class Namespace(ABC):
     @abstractmethod
     def resolved(self, name: str) -> Symbol:
diff --git a/sleepy/program/representation.py b/sleepy/program/representation.py
index f73a007..a251a42 100644
--- a/sleepy/program/representation.py
+++ b/sleepy/program/representation.py
@@ -1,12 +1,14 @@
 from dataclasses import dataclass
+from typing import override
 
-SymbolId = int
+from sleepy.core import UID, Identifiable
 
 
-class ProgramNode:
+class ProgramNode(Identifiable):
     @property
-    def uid(self) -> SymbolId:
-        return id(self)
+    @override
+    def uid(self) -> UID:
+        return UID(id(self))
 
 
 class Expression(ProgramNode):
diff --git a/sleepy/syntax/s2p.py b/sleepy/syntax/s2p.py
index 94849b8..0656950 100644
--- a/sleepy/syntax/s2p.py
+++ b/sleepy/syntax/s2p.py
@@ -55,9 +55,7 @@ def visit_conditional(self, tree: IfExpressionAST) -> Conditional:
     @override
     def visit_application(self, tree: ApplicationAST) -> ProgramNode:
         return Application(
-            invokable=self.visit_expression(
-                tree.invokable.expression,
-            ),
+            invokable=self.visit_expression(tree.invokable.expression),
             args=[self.visit_expression(arg) for arg in tree.args.expressions],
         )
 
@@ -81,10 +79,7 @@ def visit_lambda(self, tree: LambdaAST) -> Closure:
             self.bindings.bind(parameter.name, parameter)
 
         closure = Closure(parameters, statements=[])
-        self.bindings.bind(
-            self.namespace.define(Symbol("self")),
-            closure,
-        )
+        self.bindings.bind(self.namespace.define(Symbol("self")), closure)
 
         closure.statements = [
             self.visit_expression(expression)
@@ -103,7 +98,8 @@ def visit_kind(self, tree: KindAST) -> Kind:
         match tree.name.name:
             case "int":
                 return Kind("int")
-        raise NotImplementedError
+            case _:
+                raise NotImplementedError
 
     @override
     def visit_integer(self, tree: IntegerAST) -> Integer:
diff --git a/sleepy/tafka/emit.py b/sleepy/tafka/emit.py
index 933883d..f50f916 100644
--- a/sleepy/tafka/emit.py
+++ b/sleepy/tafka/emit.py
@@ -1,5 +1,5 @@
 from collections.abc import Generator
-from typing import override
+from typing import TYPE_CHECKING, override
 
 from sleepy.program import (
     Application,
@@ -11,7 +11,6 @@
     Kind,
     Program,
     Symbol,
-    SymbolId,
     Visitor,
 )
 from sleepy.program.unit import ProgramUnit
@@ -40,6 +39,9 @@
 from sleepy.tafka.representation import Kind as TafKind
 from sleepy.tafka.representation.rvalue import And, Invokation, Or
 
+if TYPE_CHECKING:
+    from sleepy.core import UID
+
 UniqueNameSequence = Generator[str, None, None]
 
 
@@ -52,7 +54,7 @@ def __init__(self, unit: ProgramUnit) -> None:
         self.var_names = map(str, range(10000000))
         self.lbl_names = map(str, range(10000000))
 
-        self.vars: dict[SymbolId, Var] = {}
+        self.vars: dict[UID, Var] = {}
         self.procedures: list[Procedure] = []
 
         self.current_block = self.main