diff --git a/stl/ast.py b/stl/ast.py
index eddf9ed..e875df2 100644
--- a/stl/ast.py
+++ b/stl/ast.py
@@ -89,6 +89,10 @@ class AST(object):
         # TODO: this is hack to flatten the AST. Fix!
         return phi
 
+    def __hash__(self):
+        # TODO: compute hash based on contents
+        return hash(repr(self))
+
 
 class _Top(AST):
     __slots__ = ()
@@ -96,10 +100,6 @@ class _Top(AST):
     def __repr__(self):
         return "⊤"
 
-    def __hash__(self):
-        # TODO: compute hash based on contents
-        return hash(repr(self))
-
     def __invert__(self):
         return BOT
 
@@ -110,10 +110,6 @@ class _Bot(AST):
     def __repr__(self):
         return "⊥"
 
-    def __hash__(self):
-        # TODO: compute hash based on contents
-        return hash(repr(self))
-
     def __invert__(self):
         return TOP
 
diff --git a/stl/test_params.py b/stl/test_params.py
index c6ca224..a29b097 100644
--- a/stl/test_params.py
+++ b/stl/test_params.py
@@ -1,4 +1,5 @@
 import stl
+from stl.hypothesis import SignalTemporalLogicStrategy
 
 import hypothesis.strategies as st
 from hypothesis import given
@@ -12,3 +13,8 @@ def test_params1(a, b, c):
     phi2 = phi.set_params({'a?': a, 'b?': b, 'c?': c})
     assert phi2.params == set()
     assert phi2 == stl.parse(f'G[{a}, {b}](x > {c})')
+
+
+@given(SignalTemporalLogicStrategy)
+def test_hash_stable(phi):
+    assert hash(phi) == hash(stl.parse(str(phi)))
diff --git a/stl/test_utils.py b/stl/test_utils.py
index 465a93e..fe09a5a 100644
--- a/stl/test_utils.py
+++ b/stl/test_utils.py
@@ -2,6 +2,7 @@ import stl
 from stl.hypothesis import SignalTemporalLogicStrategy
 
 from hypothesis import given
+from pytest import raises
 
 
 CONTEXT = {
@@ -23,6 +24,11 @@ def test_f_neg_or_canonical_form(phi):
         isinstance(x, (stl.ast.G, stl.ast.And)) for x in phi2.walk())
 
 
+def test_f_neg_or_canonical_form_not_implemented():
+    with raises(NotImplementedError):
+        stl.utils.f_neg_or_canonical_form(stl.ast.AST())
+
+
 def test_inline_context_rigid():
     phi = stl.parse('G(AP1)')
     phi2 = phi.inline_context(CONTEXT)