+ size_t idx = elem_idx (bit_idx);
+ elem_type mask = bit_mask (bit_idx);
+
+ /* This is equivalent to `b->bits[idx] &= ~mask' except that it
+ is guaranteed to be atomic on a uniprocessor machine. See
+ the description of the AND instruction in [IA32-v2a]. */
+ asm ("and %0, %1" : "=m" (b->bits[idx]) : "r" (~mask) : "cc");