diff --git a/parser/nodes.h b/parser/nodes.h
index 52fb5c9..0abcfce 100644
--- a/parser/nodes.h
+++ b/parser/nodes.h
@@ -15,31 +15,31 @@ class Node
         void addChildren(refptr<Node> other);
         std::vector< refptr<Node> > & getChildren() { return m_children; }
 
-        virtual int getInteger() const
+        virtual int getInteger()
         {
             std::cerr << "Warning: Node::getInteger() called!" << std::endl;
             return 0;
         }
-        virtual double getNumber() const
+        virtual double getNumber()
         {
             std::cerr << "Warning: Node::getNumber() called!" << std::endl;
             return 0.0;
         }
-        virtual refptr<Vector> getVector() const
+        virtual refptr<Vector> getVector()
         {
             std::cerr << "Warning: Node::getVector() called!" << std::endl;
             return refptr<Vector>(NULL);
         }
-        virtual std::string getString() const
+        virtual std::string getString()
         {
             std::cerr << "Warning: Node::getString() called!" << std::endl;
             return "";
         }
 
-        virtual bool isShape() const { return false; }
-        virtual bool isMaterial() const { return false; }
-        virtual bool isTransformBlock() const { return false; }
-        virtual bool isExpression() const { return false; }
+        virtual bool isShape() { return false; }
+        virtual bool isMaterial() { return false; }
+        virtual bool isTransformBlock() { return false; }
+        virtual bool isExpression() { return false; }
 
     protected:
         std::vector< refptr<Node> > m_children;
@@ -50,8 +50,8 @@ class IntegerNode : public Node
 {
     public:
         IntegerNode(int number) { m_number = number; }
-        int getInteger() const { return m_number; }
-        double getNumber() const { return m_number; }
+        virtual int getInteger() { return m_number; }
+        virtual double getNumber() { return m_number; }
 
     protected:
         int m_number;
@@ -70,7 +70,7 @@ class NumberNode : public Node
             addChild(expr);
             m_direct = false;
         }
-        double getNumber() const
+        virtual double getNumber()
         {
             return m_direct ? m_number : m_children[0]->getNumber();
         }
@@ -93,7 +93,7 @@ class VectorNode : public Node
         {
             addChild(node);
         }
-        refptr<Vector> getVector() const
+        refptr<Vector> getVector()
         {
             if (m_children.size() == 1)
             {
@@ -177,7 +177,7 @@ class IdentifierNode : public Node
 {
     public:
         IdentifierNode(const std::string & str) { m_string = str; }
-        std::string getString() const
+        std::string getString()
         {
             return m_string;
         }
@@ -272,7 +272,7 @@ class PlanePositionNode : public VectorNode
         {
             m_dist = dist;
         }
-        double getNumber() const { return m_dist->getNumber(); }
+        double getNumber() { return m_dist->getNumber(); }
 
     protected:
         refptr<Node> m_dist;
@@ -430,8 +430,9 @@ class WidthNode : public IntegerNode
 class ExpressionNode : public Node
 {
     public:
-        bool isExpression() const { return true; }
-        virtual int getInteger() const { return getNumber(); }
+        bool isExpression() { return true; }
+        virtual double getNumber() = 0;
+        virtual int getInteger() { return getNumber(); }
 };
 
 class BinOpNode : public ExpressionNode