diff --git a/mythril/laser/ethereum/instructions.py b/mythril/laser/ethereum/instructions.py
index 1fd74673..82a3d160 100644
--- a/mythril/laser/ethereum/instructions.py
+++ b/mythril/laser/ethereum/instructions.py
@@ -4,7 +4,7 @@ import binascii
 import logging
 
 from copy import copy, deepcopy
-from typing import cast, Callable, List, Union, Tuple
+from typing import cast, Callable, List, Set, Union, Tuple, Any
 from datetime import datetime
 from math import ceil
 from ethereum import utils
@@ -553,7 +553,7 @@ class Instruction:
                     + str(hash(simplify(exponent)))
                     + ")",
                     256,
-                    base.annotations + exponent.annotations,
+                    base.annotations.union(exponent.annotations),
                 )
             )  # Hash is used because str(symbol) takes a long time to be converted to a string
         else:
@@ -562,7 +562,7 @@ class Instruction:
                 symbol_factory.BitVecVal(
                     pow(base.value, exponent.value, 2 ** 256),
                     256,
-                    annotations=base.annotations + exponent.annotations,
+                    annotations=base.annotations.union(exponent.annotations),
                 )
             )
 
@@ -925,11 +925,11 @@ class Instruction:
 
         if data.symbolic:
 
-            annotations = []
+            annotations = set()  # type: Set[Any]
 
             for b in state.memory[index : index + length]:
                 if isinstance(b, BitVec):
-                    annotations += b.annotations
+                    annotations = annotations.union(b.annotations)
 
             argument_str = str(state.memory[index]).replace(" ", "_")
             result = symbol_factory.BitVecFuncSym(
diff --git a/mythril/laser/smt/__init__.py b/mythril/laser/smt/__init__.py
index 5a3f1ca1..407f6565 100644
--- a/mythril/laser/smt/__init__.py
+++ b/mythril/laser/smt/__init__.py
@@ -22,11 +22,11 @@ from mythril.laser.smt.array import K, Array, BaseArray
 from mythril.laser.smt.solver import Solver, Optimize, SolverStatistics
 from mythril.laser.smt.model import Model
 
-from typing import Union, Any, Optional, List, TypeVar, Generic
+from typing import Union, Any, Optional, Set, TypeVar, Generic
 import z3
 
 
-Annotations = Optional[List[Any]]
+Annotations = Optional[Set[Any]]
 T = TypeVar("T", bound=Union[bool.Bool, z3.BoolRef])
 U = TypeVar("U", bound=Union[BitVec, z3.BitVecRef])
 
diff --git a/mythril/laser/smt/bitvec.py b/mythril/laser/smt/bitvec.py
index 1dd15191..11060d04 100644
--- a/mythril/laser/smt/bitvec.py
+++ b/mythril/laser/smt/bitvec.py
@@ -1,13 +1,13 @@
 """This module provides classes for an SMT abstraction of bit vectors."""
 
-from typing import Union, overload, List, cast, Any, Optional, Callable
+from typing import Union, overload, List, Set, cast, Any, Optional, Callable
 from operator import lshift, rshift
 import z3
 
 from mythril.laser.smt.bool import Bool, And, Or
 from mythril.laser.smt.expression import Expression
 
-Annotations = List[Any]
+Annotations = Set[Any]
 
 # fmt: off
 
@@ -61,7 +61,7 @@ class BitVec(Expression[z3.BitVecRef]):
         if isinstance(other, int):
             return BitVec(self.raw + other, annotations=self.annotations)
 
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw + other.raw, annotations=union)
 
     def __sub__(self, other: Union[int, "BitVec"]) -> "BitVec":
@@ -75,7 +75,7 @@ class BitVec(Expression[z3.BitVecRef]):
         if isinstance(other, int):
             return BitVec(self.raw - other, annotations=self.annotations)
 
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw - other.raw, annotations=union)
 
     def __mul__(self, other: "BitVec") -> "BitVec":
@@ -86,7 +86,7 @@ class BitVec(Expression[z3.BitVecRef]):
         """
         if isinstance(other, BitVecFunc):
             return other * self
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw * other.raw, annotations=union)
 
     def __truediv__(self, other: "BitVec") -> "BitVec":
@@ -97,7 +97,7 @@ class BitVec(Expression[z3.BitVecRef]):
         """
         if isinstance(other, BitVecFunc):
             return other / self
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw / other.raw, annotations=union)
 
     def __and__(self, other: Union[int, "BitVec"]) -> "BitVec":
@@ -110,7 +110,7 @@ class BitVec(Expression[z3.BitVecRef]):
             return other & self
         if not isinstance(other, BitVec):
             other = BitVec(z3.BitVecVal(other, self.size()))
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw & other.raw, annotations=union)
 
     def __or__(self, other: "BitVec") -> "BitVec":
@@ -121,7 +121,7 @@ class BitVec(Expression[z3.BitVecRef]):
         """
         if isinstance(other, BitVecFunc):
             return other | self
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw | other.raw, annotations=union)
 
     def __xor__(self, other: "BitVec") -> "BitVec":
@@ -132,7 +132,7 @@ class BitVec(Expression[z3.BitVecRef]):
         """
         if isinstance(other, BitVecFunc):
             return other ^ self
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(self.raw ^ other.raw, annotations=union)
 
     def __lt__(self, other: "BitVec") -> Bool:
@@ -143,7 +143,7 @@ class BitVec(Expression[z3.BitVecRef]):
         """
         if isinstance(other, BitVecFunc):
             return other > self
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return Bool(self.raw < other.raw, annotations=union)
 
     def __gt__(self, other: "BitVec") -> Bool:
@@ -154,7 +154,7 @@ class BitVec(Expression[z3.BitVecRef]):
         """
         if isinstance(other, BitVecFunc):
             return other < self
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return Bool(self.raw > other.raw, annotations=union)
 
     def __le__(self, other: "BitVec") -> Bool:
@@ -163,7 +163,7 @@ class BitVec(Expression[z3.BitVecRef]):
         :param other:
         :return:
         """
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return Bool(self.raw <= other.raw, annotations=union)
 
     def __ge__(self, other: "BitVec") -> Bool:
@@ -172,7 +172,7 @@ class BitVec(Expression[z3.BitVecRef]):
         :param other:
         :return:
         """
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return Bool(self.raw >= other.raw, annotations=union)
 
     # MYPY: fix complains about overriding __eq__
@@ -189,7 +189,7 @@ class BitVec(Expression[z3.BitVecRef]):
                 cast(z3.BoolRef, self.raw == other), annotations=self.annotations
             )
 
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         # MYPY: fix complaints due to z3 overriding __eq__
         return Bool(cast(z3.BoolRef, self.raw == other.raw), annotations=union)
 
@@ -207,7 +207,7 @@ class BitVec(Expression[z3.BitVecRef]):
                 cast(z3.BoolRef, self.raw != other), annotations=self.annotations
             )
 
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         # MYPY: fix complaints due to z3 overriding __eq__
         return Bool(cast(z3.BoolRef, self.raw != other.raw), annotations=union)
 
@@ -224,7 +224,7 @@ class BitVec(Expression[z3.BitVecRef]):
             return BitVec(
                 operator(self.raw, other), annotations=self.annotations
             )
-        union = self.annotations + other.annotations
+        union = self.annotations.union(other.annotations)
         return BitVec(operator(self.raw, other.raw), annotations=union)
 
     def __lshift__(self, other: Union[int, "BitVec"]) -> "BitVec":
@@ -254,7 +254,7 @@ class BitVec(Expression[z3.BitVecRef]):
 def _comparison_helper(
     a: BitVec, b: BitVec, operation: Callable, default_value: bool, inputs_equal: bool
 ) -> Bool:
-    annotations = a.annotations + b.annotations
+    annotations = a.annotations.union(b.annotations)
     if isinstance(a, BitVecFunc):
         if not a.symbolic and not b.symbolic:
             return Bool(operation(a.raw, b.raw), annotations=annotations)
@@ -277,7 +277,7 @@ def _comparison_helper(
 
 def _arithmetic_helper(a: BitVec, b: BitVec, operation: Callable) -> BitVec:
     raw = operation(a.raw, b.raw)
-    union = a.annotations + b.annotations
+    union = a.annotations.union(b.annotations)
 
     if isinstance(a, BitVecFunc) and isinstance(b, BitVecFunc):
         return BitVecFunc(raw=raw, func_name=None, input_=None, annotations=union)
@@ -313,7 +313,7 @@ def If(a: Union[Bool, bool], b: Union[BitVec, int], c: Union[BitVec, int]) -> Bi
         b = BitVec(z3.BitVecVal(b, 256))
     if not isinstance(c, BitVec):
         c = BitVec(z3.BitVecVal(c, 256))
-    union = a.annotations + b.annotations + c.annotations
+    union = a.annotations.union(b.annotations).union(c.annotations)
     return BitVec(z3.If(a.raw, b.raw, c.raw), union)
 
 
@@ -378,10 +378,10 @@ def Concat(*args: Union[BitVec, List[BitVec]]) -> BitVec:
         bvs = cast(List[BitVec], args)
 
     nraw = z3.Concat([a.raw for a in bvs])
-    annotations = []  # type: Annotations
+    annotations = set()  # type: Annotations
     bitvecfunc = False
     for bv in bvs:
-        annotations += bv.annotations
+        annotations = annotations.union(bv.annotations)
         if isinstance(bv, BitVecFunc):
             bitvecfunc = True
 
@@ -448,11 +448,11 @@ def Sum(*args: BitVec) -> BitVec:
     :return:
     """
     raw = z3.Sum([a.raw for a in args])
-    annotations = []  # type: Annotations
+    annotations = set()  # type: Annotations
     bitvecfuncs = []
 
     for bv in args:
-        annotations += bv.annotations
+        annotations = annotations.union(bv.annotations)
         if isinstance(bv, BitVecFunc):
             bitvecfuncs.append(bv)
 
diff --git a/mythril/laser/smt/bitvecfunc.py b/mythril/laser/smt/bitvecfunc.py
index 257d2b46..8d680648 100644
--- a/mythril/laser/smt/bitvecfunc.py
+++ b/mythril/laser/smt/bitvecfunc.py
@@ -23,7 +23,7 @@ def _arithmetic_helper(
         b = BitVec(z3.BitVecVal(b, a.size()))
 
     raw = operation(a.raw, b.raw)
-    union = a.annotations + b.annotations
+    union = a.annotations.union(b.annotations)
 
     if isinstance(b, BitVecFunc):
         # TODO: Find better value to set input and name to in this case?
@@ -53,7 +53,7 @@ def _comparison_helper(
     if isinstance(b, int):
         b = BitVec(z3.BitVecVal(b, a.size()))
 
-    union = a.annotations + b.annotations
+    union = a.annotations.union(b.annotations)
 
     if not a.symbolic and not b.symbolic:
         return Bool(z3.BoolVal(operation(a.value, b.value)), annotations=union)
diff --git a/mythril/laser/smt/bool.py b/mythril/laser/smt/bool.py
index 40f098fc..c09358d0 100644
--- a/mythril/laser/smt/bool.py
+++ b/mythril/laser/smt/bool.py
@@ -1,7 +1,7 @@
 """This module provides classes for an SMT abstraction of boolean
 expressions."""
 
-from typing import Union, cast, List
+from typing import Union, cast, List, Set
 
 import z3
 
@@ -55,7 +55,7 @@ class Bool(Expression[z3.BoolRef]):
         """
         if isinstance(other, Expression):
             return Bool(cast(z3.BoolRef, self.raw == other.raw),
-                        self.annotations + other.annotations)
+                        self.annotations.union(other.annotations))
         return Bool(cast(z3.BoolRef, self.raw == other), self.annotations)
 
     # MYPY: complains about overloading __ne__ # noqa
@@ -67,7 +67,7 @@ class Bool(Expression[z3.BoolRef]):
         """
         if isinstance(other, Expression):
             return Bool(cast(z3.BoolRef, self.raw != other.raw),
-                        self.annotations + other.annotations)
+                        self.annotations.union(other.annotations))
         return Bool(cast(z3.BoolRef, self.raw != other), self.annotations)
 
     def __bool__(self) -> bool:
@@ -86,17 +86,17 @@ class Bool(Expression[z3.BoolRef]):
 
 def And(*args: Union[Bool, bool]) -> Bool:
     """Create an And expression."""
-    union = []    # type: List
+    annotations = set()   # type: Set
     args_list = [arg if isinstance(arg, Bool) else Bool(arg) for arg in args]
     for arg in args_list:
-        union += arg.annotations
-    return Bool(z3.And([a.raw for a in args_list]), union)
+        annotations = annotations.union(arg.annotations)
+    return Bool(z3.And([a.raw for a in args_list]), annotations)
 
 
 def Xor(a: Bool, b: Bool) -> Bool:
     """Create an And expression."""
 
-    union = a.annotations + b.annotations
+    union = a.annotations.union(b.annotations)
     return Bool(z3.Xor(a.raw, b.raw), union)
 
 
@@ -108,10 +108,10 @@ def Or(*args: Union[Bool, bool]) -> Bool:
     :return:
     """
     args_list = [arg if isinstance(arg, Bool) else Bool(arg) for arg in args]
-    union = []  # type: List
+    annotations = set()  # type: Set
     for arg in args_list:
-        union += arg.annotations
-    return Bool(z3.Or([a.raw for a in args_list]), annotations=union)
+        annotations = annotations.union(arg.annotations)
+    return Bool(z3.Or([a.raw for a in args_list]), annotations=annotations)
 
 
 def Not(a: Bool) -> Bool:
diff --git a/mythril/laser/smt/expression.py b/mythril/laser/smt/expression.py
index ae98cd31..9fa7cef1 100644
--- a/mythril/laser/smt/expression.py
+++ b/mythril/laser/smt/expression.py
@@ -1,9 +1,9 @@
 """This module contains the SMT abstraction for a basic symbol expression."""
-from typing import Optional, List, Any, TypeVar, Generic, cast
+from typing import Optional, Set, Any, TypeVar, Generic, cast
 import z3
 
 
-Annotations = List[Any]
+Annotations = Set[Any]
 T = TypeVar("T", bound=z3.ExprRef)
 
 
@@ -18,7 +18,11 @@ class Expression(Generic[T]):
         :param annotations: 
         """
         self.raw = raw
-        self._annotations = annotations or []
+
+        if annotations:
+            assert isinstance(annotations, set)
+
+        self._annotations = annotations or set()
 
     @property
     def annotations(self) -> Annotations:
@@ -26,6 +30,7 @@ class Expression(Generic[T]):
 
         :return:
         """
+
         return self._annotations
 
     def annotate(self, annotation: Any) -> None:
@@ -33,10 +38,8 @@ class Expression(Generic[T]):
 
         :param annotation:
         """
-        if isinstance(annotation, list):
-            self._annotations += annotation
-        else:
-            self._annotations.append(annotation)
+
+        self._annotations.add(annotation)
 
     def simplify(self) -> None:
         """Simplify this expression."""