Add booleans

tree.hpp

@@ -41,6 +41,14 @@ struct expr_id : expr {
     virtual void print_dot(dot& into, std::string prefix);
 };
 
+struct expr_bool : expr {
+    bool val;
+
+    expr_bool(bool v) : val(v) {}
+
+    virtual void print_dot(dot& into, std::string prefix);
+};
+
 struct expr_int : expr {
     int val;