Implement clone for the context.

src/parser/list.rs

@@ -1,9 +1,11 @@
 use std::rc::Rc;
 
+#[derive(Debug, Clone)]
 pub struct List<T> {
     head: Option<Rc<Node<T>>>,
 }
 
+#[derive(Debug, Clone)]
 pub struct Node<T> {
     data: T,
     parent: Option<Rc<Node<T>>>,