added test coverage for f_neg_canonical_form

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)))