question on bitmap_set_subtract unction in pre

[Amker.Cheng]

Hi,
In PRE, function compute_antic_aux uses bitmap_set_subtract to compute
value/expression set subtraction.

The comment of bitmap_set_subtract says it subtracts all the values
and expressions contained in ORIG from DEST.

But the implementation as following
---------------------------------------------------
static bitmap_set_t
bitmap_set_subtract (bitmap_set_t dest, bitmap_set_t orig)
{
  bitmap_set_t result = bitmap_set_new ();
  bitmap_iterator bi;
  unsigned int i;

  bitmap_and_compl (&result->expressions, &dest->expressions,
                    &orig->expressions);

  FOR_EACH_EXPR_ID_IN_SET (result, i, bi)
    {
      pre_expr expr = expression_for_id (i);
      unsigned int value_id = get_expr_value_id (expr);
      bitmap_set_bit (&result->values, value_id);
    }

  return result;
}

Does it just subtract the expressions, rather than values. And It
resets values according to the resulting expression.

I am a little confused here. Any explanation?

Thanks very much.
-- 
Best Regards.