- If func is non-zero, undoing this binding applies func to old_value;
- This implements record_unwind_protect.
-
- Otherwise, the element is a variable binding.
-
- If the symbol field is a symbol, it is an ordinary variable binding.
-
- Otherwise, it should be a structure (SYMBOL WHERE . CURRENT-BUFFER),
- which means having bound a local value while CURRENT-BUFFER was active.
- If WHERE is nil this means we saw the default value when binding SYMBOL.
- WHERE being a buffer or frame means we saw a buffer-local or frame-local
- value. Other values of WHERE mean an internal error.
-