diff --git a/kr/src/ast.rs b/kr/src/ast.rs
index 88494ea..c6b2e0c 100644
--- a/kr/src/ast.rs
+++ b/kr/src/ast.rs
@@ -55,5 +55,12 @@ impl Form {
             _ => None,
         }
     }
+    pub fn append(&self, x: Rc<Form>) -> Option<Form> {
+        match self {
+            Form::Pair(car, cdr) => cdr.append(x).map(|x| Form::Pair(Rc::clone(car), Rc::new(x))),
+            Form::Nil            => Some(Form::Pair(x, Rc::new(Form::Nil))),
+            _                    => None,
+        }
+    }
 }
 
diff --git a/kr/src/grammar.lalrpop b/kr/src/grammar.lalrpop
index 266de55..21c51ce 100644
--- a/kr/src/grammar.lalrpop
+++ b/kr/src/grammar.lalrpop
@@ -9,6 +9,9 @@ pub Term: Form = {
     SYM => Form::Symbol(<>.to_owned()),
 	"(" <ListInside?> ")" => <>.unwrap_or(Form::Nil),
 	"'" <Term> => Form::Pair(Rc::new(Form::Symbol("quote".to_owned())), Rc::new(Form::Pair(Rc::new(<>), Rc::new(Form::Nil)))),
+	"!" <h: Term> <t: Term> => {
+        h.append(Rc::new(t)).unwrap()
+     },
 };
 ListInside: Form = {
 	<Term> => Form::Pair(Rc::new(<>), Rc::new(Form::Nil)),
@@ -20,6 +23,7 @@ match {
     ")",
     ".",
     "'",
+    "!",
     r"[0-9]+"                              => NUM,
     r"[a-zA-Z+*/_=?%&|^-][\w+*/=_?%&|^-]*" => SYM,
     r"(;[^\n]*\n)|\s+"                     => { }
diff --git a/kr/src/main.rs b/kr/src/main.rs
index 23e19b1..7dc0035 100644
--- a/kr/src/main.rs
+++ b/kr/src/main.rs
@@ -62,16 +62,17 @@ fn parse_test() {
     eval_test(&g, &e, "(bool? true)", true);
     eval_test(&g, &e, "(bool? 1)", false);
 
+    eval_test(&g, &e, "!(bool?) 1",    false);
+    eval_test(&g, &e, "!(bool?) true", true);
+
     eval_test(&g, &e, "((vau root_env _ (eval 'a (cons (cons 'a 2) root_env))))", 2);
     let LET = "
-    ((vau root_env _ (eval '(let1 a 8 (+ a 9))
+    !((vau root_env p (eval (car p)
       (cons (cons 'let1
+     (vau de p (eval (car (cdr (cdr p))) (cons (cons (car p) (eval (car (cdr p)) de)) de)))
+    ) root_env))))";
 
-     (vau de p (eval (car (cdr (cdr p))) (cons (cons (car p) (eval (car (cdr p)) de)) de))) 
-    ) root_env))))
-    ";
-
-    eval_test(&g, &e, LET, 17);
+    eval_test(&g, &e, &format!("{} (let1 x 10 (+ x 7))", LET), 17);
 
     // TODO, finish lambda
     let LAMBDA = "