add more unit tests for function calls

src/segment.rs

@@ -23,6 +23,7 @@ pub type Ast = Rc<RefCell<Seg>>;
 
 // Container
 #[derive(Clone)]
+#[derive(Debug)]
 pub enum Ctr {
     Symbol(String),
     String(String),
@@ -53,6 +54,7 @@ pub enum Type {
  * how important RefCells were in Rust
  */
 #[derive(Clone)]
+#[derive(Debug)]
 pub struct Seg {
     /* "Contents of Address Register"
      *  Historical way of referring to the first value in a cell.
@@ -215,7 +217,7 @@ pub fn list_idx(tree: Ast, idx: u128) -> Ctr {
         match inner.car {
             Ctr::None => Ctr::None,
             _ => {
-                inner.cdr.clone()
+                inner.car.clone()
             }
         }
     }