diff --git a/lib/src/adf.rs b/lib/src/adf.rs
index b0e8487..92ab300 100644
--- a/lib/src/adf.rs
+++ b/lib/src/adf.rs
@@ -614,7 +614,6 @@ mod test {
         let _adf = Adf::default();
     }
 
-    #[cfg(feature = "variablelist")]
     #[test]
     fn facet_counts() {
         let parser = AdfParser::default();
diff --git a/lib/src/datatypes/bdd.rs b/lib/src/datatypes/bdd.rs
index c313008..2f1dedb 100644
--- a/lib/src/datatypes/bdd.rs
+++ b/lib/src/datatypes/bdd.rs
@@ -113,6 +113,11 @@ impl Var {
     pub fn value(self) -> usize {
         self.0
     }
+
+    /// Returns true if the value of the variable is a constant (i.e. Top or Bot)
+    pub fn is_constant(&self) -> bool {
+        self.value() >= Var::BOT.value()
+    }
 }
 
 /// A [BddNode] is representing one Node in the decision diagram
@@ -216,6 +221,17 @@ mod test {
         assert_eq!(*node.var(), var);
         assert_eq!(*node.lo(), lo);
         assert_eq!(*node.hi(), hi);
+        match node.var() {
+            Var::TOP => {
+                assert!(node.var().is_constant());
+            }
+            Var::BOT => {
+                assert!(node.var().is_constant());
+            }
+            val => {
+                assert!(!val.is_constant());
+            }
+        }
         true
     }
 }
diff --git a/lib/src/obdd.rs b/lib/src/obdd.rs
index 008920a..d22a61d 100644
--- a/lib/src/obdd.rs
+++ b/lib/src/obdd.rs
@@ -323,10 +323,18 @@ impl Bdd {
         }
         #[cfg(not(feature = "variablelist"))]
         {
-            let _ = tree;
-            HashSet::new()
+            let node = self.nodes[tree.value()];
+            if node.var().is_constant() {
+                return HashSet::new();
+            }
+            let mut var_set = self
+                .var_dependencies(node.lo())
+                .union(&self.var_dependencies(node.hi()))
+                .copied()
+                .collect::<HashSet<Var>>();
+            var_set.insert(node.var());
+            var_set
         }
-        // TODO!
     }
 }