void mergeSort(vector  & a, int first, int last)
{
   int mid;
   
   if ( first == last);    // empty case, only 1 value, do nothing
   else
      if (first+1 == last) // list of 2 values, swap if necessary
      {
         if (a[first] > a[last])
            swap( a[first], a[last] );
      }
      else    // general case
      {
         mid = (first + last)/2;
         mergeSort(a, first, mid);
         mergeSort(a, mid+1, last);
         merge(a, first, mid, last);
      }
}