diff --git a/stl/__init__.py b/stl/__init__.py
index e9e248a..e0b5059 100644
--- a/stl/__init__.py
+++ b/stl/__init__.py
@@ -1,4 +1,4 @@
-from stl.utils import terms_lens, lineq_lens, walk, tree, and_or_lens
+from stl.utils import terms_lens, lineq_lens, walk, and_or_lens
 from stl.utils import alw, env, andf, orf
 from stl.ast import dt_sym, t_sym
 from stl.ast import LinEq, Interval, NaryOpSTL, Or, And, F, G, ModalOp, Neg, Var, AtomicPred
diff --git a/stl/test_parser.py b/stl/test_parser.py
index 480e4d3..51208a5 100644
--- a/stl/test_parser.py
+++ b/stl/test_parser.py
@@ -5,17 +5,17 @@ import unittest
 from sympy import Symbol
 
 ex1 = ('x1 > 2', stl.LinEq(
-    (stl.Var(1, Symbol("x1"), stl.ast.t_sym),),
+    (stl.Var(1, Symbol("x1"), stl.t_sym),),
     ">",
     2.0
 ))
 ex1_ = ('x1 > a?', stl.LinEq(
-    (stl.Var(1, Symbol("x1"), stl.ast.t_sym),),
+    (stl.Var(1, Symbol("x1"), stl.t_sym),),
     ">",
     Symbol("a?")
 ))
 
-ex1__ = ('x1', stl.AtomicPred('x1'))
+ex1__ = ('x1', stl.AtomicPred('x1', stl.t_sym))
 
 i1 = stl.Interval(0., 1.)
 i1_ = stl.Interval(0., Symbol("b?"))
diff --git a/stl/test_utils.py b/stl/test_utils.py
index f417d4c..a2c78fc 100644
--- a/stl/test_utils.py
+++ b/stl/test_utils.py
@@ -28,3 +28,31 @@ class TestSTLUtils(unittest.TestCase):
         
         self.assertEqual(set(map(str, stl.utils.param_lens(phi).get_all())), set())
         self.assertEqual(phi, phi2)
+
+    def test_walk(self):
+        raise NotImplementedError
+
+
+    def test_type_pred(self):
+        raise NotImplementedError
+
+
+    def test_ast_lens(self):
+        raise NotImplementedError
+
+
+    def test_terms_lens(self):
+        raise NotImplementedError
+
+
+    def test_f_neg_or_canonical_form(self):
+        raise NotImplementedError
+
+    def test_to_from_mtl(self):
+        raise NotImplementedError
+
+    def test_get_polarity(self):
+        raise NotImplementedError
+
+    def test_canonical_polarity(self):
+        raise NotImplementedError
diff --git a/stl/utils.py b/stl/utils.py
index 19e7bf2..0209c0a 100644
--- a/stl/utils.py
+++ b/stl/utils.py
@@ -22,10 +22,6 @@ def walk(phi:STL, bfs:bool=False) -> STL_Generator:
         children.extend(node.children())
 
 
-def tree(phi:STL) -> Dict[STL, STL]:
-    return {x:set(x.children()) for x in walk(phi) if x.children()}
-
-
 def type_pred(*args:List[Type]) -> Mapping[Type, bool]:
     ast_types = set(args)
     return lambda x: type(x) in ast_types
@@ -80,16 +76,6 @@ def param_lens(phi:STL) -> Lens:
                     focus_lens=focus_lens).filter_(is_sym)
 
 
-def symbol_lens(phi:STL) -> Lens:
-    is_sym = lambda x: isinstance(x, sympy.Symbol)
-    def focus_lens(leaf):
-        spacial = [lens().const] + lens().terms.each_().id.get_all()
-        temporal = [lens().interval[0], lens().interval[1]]
-        return spacial if isinstance(leaf, LinEq) else temp
-
-    return ast_lens(phi, pred=type_pred(LinEq, F, G), 
-                    focus_lens=focus_lens).filter_(is_sym)
-
 def set_params(stl_or_lens, val) -> STL:
     l = stl_or_lens if isinstance(stl_or_lens, Lens) else param_lens(stl_or_lens)
     return l.modify(lambda x: val.get(x, val.get(str(x), x)))
@@ -131,6 +117,13 @@ def from_mtl(phi:MTL, ap_map:Dict[AtomicPred, LinEq]) -> STL:
     return focus.modify(ap_map.get)
 
 
+def get_polarity(phi, traces=None):
+    raise NotImplementedError
+
+def canonical_polarity(phi, traces=None):
+    raise NotImplementedError
+
+
 # EDSL
 
 def alw(phi, *, lo, hi):