diff --git a/sources/zweic/Tree.scala b/sources/zweic/Tree.scala
index f081ed4..e320a6f 100755
--- a/sources/zweic/Tree.scala
+++ b/sources/zweic/Tree.scala
@@ -78,48 +78,32 @@
 //############################################################################
 // Tree nodes for statements
 
-/**
- * A common superclass for all statements
- */
+/* A common superclass for all statements */
 abstract class Stat extends Tree;
 
-/**
- * S = While E { S }
- */
+/* S = While E { S } */
 case class While(cond: Expr, stats: List[Stat]) extends Stat;
 
-/**
- * S = Var name T E
- */
+/* S = Var name T E */
 case class Var(varname: Name, vartype: TypeTree, init: Expr)
      extends Stat;
 
-/**
- * S = Set name E
- */
+/* S = Set name E */
 case class Set(name: Name, expr: Expr) extends Stat;
 
-/**
- * S = Do E
- */
+/* S = Do E */
 case class Do(expr: Expr) extends Stat;
 
-/**
- * S = PrintInt E
- */
+/* S = PrintInt E */
 case class PrintInt(expr: Expr) extends Stat;
 
-/**
- * S = PrintChar E
- */
+/* S = PrintChar E */
 case class PrintChar(expr: Expr) extends Stat;
 
 //############################################################################
 // Tree nodes for expressions
 
-/**
- * A common superclass for expressions
- */
+/* A common superclass for expressions */
 abstract class Expr extends Tree;
 
 /* E = Ident name */
@@ -134,6 +118,12 @@
 /* E = Call E name { E } */
 case class Call(func: Expr, name: Name, args: List[Expr]) extends Expr;
 
+/* E = NullLit */
+case class NullLit extends Expr;
+
+/* E = IntLit int */
+case class IntLit(value: int) extends Expr;
+
 /* E = Unop O E */
 case class Unop(op: Operator, expr: Expr) extends Expr;
 
@@ -152,5 +142,4 @@
 /* E = Block { S } E */
 case class Block(stats: List[Stat], expression: Expr) extends Expr;
 
-
 //############################################################################