177   // It returns x - y if x > y, 0 otherwise.

   178   // As described in the comment above, some of the inputs to the

   179   // calculations we have to do are obtained concurrently and hence

   180   // may be inconsistent with each other. So, this provides a

   181   // defensive way of performing the subtraction and avoids the value

   182   // going negative (which would mean a very large result, given that

   183   // the parameter are size_t).

   184   static size_t subtract_up_to_zero(size_t x, size_t y) {

   185     if (x > y) {

   186       return x - y;

   187     } else {

   188       return 0;

   189     }

   190   }

   191