fix append func

Signed-off-by: Ava Hahn <ava@aidanis.online>

tests/test_lib_append.rs

@@ -20,6 +20,24 @@ mod append_lib_tests {
         }
     }
 
+    #[test]
+    fn test_multi_append_to_empty_list() {
+        let document = "(append () 1 'two' 3.4)";
+        let result = "(1 'two' 3.4)";
+
+        let mut syms = SymTable::new();
+        static_stdlib(&mut syms).unwrap();
+        dynamic_stdlib(false, &mut syms).unwrap();
+
+        if let Ok(tree) = lex(&document.to_string()) {
+            if let Ctr::Seg(ref s) = *eval(&tree, &mut syms).unwrap() {
+                assert_eq!(s.to_string(), result);
+            }
+        } else {
+            assert!(false)
+        }
+    }
+
     #[test]
     fn test_append_to_full_list() {
         let document = "(append (1 2) 3)";