ATTNLEX -- "Attention Key lexfile".  Adds ATTN ON / OFF to BASIC.

Size: 68 bytes.

ATTN OFF disables the ATTN key from interrupting programs; it becomes
just another key that gets put into the key buffer when pressed.

ATTN ON undoes ATTN OFF; it restores the action of the ATTN key to
normal, so that it can interrupt running programs.

Looks better than POKE "2F441","0" and POKE "2F441","1", but has
identical effects.