ANSI Common Lisp   3 Evaluation and Compilation

3.6 Traversal Rules and Side Effects

The consequences are undefined when code executed during an object-traversing operation destructively modifies the object in a way that might affect the ongoing traversal operation. In particular, the following rules apply.

• List traversal

  For list traversal operations, the cdr chain of the list is not allowed to be destructively modified.

• Array traversal

  For array traversal operations, the array is not allowed to be adjusted and its fill pointer, if any, is not allowed to be changed.

• Hash-table traversal

  For hash table traversal operations, new elements may not be added or deleted except that the element corresponding to the current hash key may be changed or removed.

• Package traversal

  For package traversal operations (e.g., do-symbols), new symbols may not be interned in or uninterned from the package being traversed or any package that it uses except that the current symbol may be uninterned from the package being traversed.