diff --git a/src/error.rs b/src/error.rs
index aaddc77..7df44c3 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -35,10 +35,9 @@ impl Traceback {
         Traceback(vec![])
     }
 
-    pub fn with_trace(&self, item: TraceItem) -> Traceback {
-        let mut next = Traceback(self.0.clone());
-        next.0.push(item);
-        next
+    pub fn with_trace(mut self, item: TraceItem) -> Traceback {
+        self.0.push(item);
+        self
     }
 
     pub fn depth(&self) -> usize {
diff --git a/src/stl/control.rs b/src/stl/control.rs
index 2544603..0d45cce 100644
--- a/src/stl/control.rs
+++ b/src/stl/control.rs
@@ -330,7 +330,7 @@ example: (circuit (eq? (do-operation) myresult)
 in this example, do-another-operation will not be called";
 fn circuit_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, Traceback> {
     let mut cursor = 0;
-    let mut err_trace = Traceback::new();
+    let mut err_trace: Traceback = Traceback::new();
     let result = ast.circuit(&mut |form: &Ctr| -> bool {
         cursor += 1;
         let operand: &Seg;
@@ -358,7 +358,7 @@ fn circuit_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, Traceback> {
                     } else if let Ctr::Integer(i) = *s.car {
                         return i==0;
                     } else {
-                        err_trace = err_trace.with_trace(
+                        err_trace = err_trace.clone().with_trace(
                             ("circuit", "impossible condition")
                                 .into());
                     }
@@ -366,7 +366,7 @@ fn circuit_callback(ast: &Seg, syms: &mut SymTable) -> Result<Ctr, Traceback> {
 
                 Ctr::Integer(i) => return i == 0,
 
-                _ => err_trace = err_trace.with_trace(
+                _ => err_trace = err_trace.clone().with_trace(
                     ("circuit", format!("form {cursor} did not evaluate to a boolean"))
                         .into()),
             },