/** Return a reference to the basic-derived class T object embedded in an
* expression. This is fast but unsafe: the result is undefined if the
* expression does not contain a T object at its top level. Hence, you
- * should generally check the type of e first.
+ * should generally check the type of e first. Also, you shouldn't cache
+ * the returned reference because GiNaC's garbage collector may destroy
+ * the referenced object any time it's used in another expression.
*
* @param e expression
* @return reference to object of class T