void BinarySearch(const int A[], int First, int Last, 
                  int Value, int& Index)
// ---------------------------------------------------------
// Searches the array items A[First] through A[Last] 
// for Value by using a binary search.
// Precondition: 0 <= First, Last <= SIZE-1, where
// SIZE is the maximum size of the array, and
// A[First] <= A[First+1] <= ... <= A[Last].
// Postcondition: If Value is in the array, Index is
// the index of the array item that equals Value;
// otherwise Index is -1.
// ---------------------------------------------------------
{
   if (First > Last)
      Index = -1;      // Value not in original array

   else
   {  // Invariant: If Value is in A, 
      //            A[First] <= Value <= A[Last]
      int Mid = (First + Last)/2;
      if (Value == A[Mid])
         Index = Mid;  // Value found at A[Mid]

      else if (Value < A[Mid])
         BinarySearch(A, First, Mid-1, Value, Index);  // X

      else
         BinarySearch(A, Mid+1, Last, Value, Index);   // Y
   }  // end else
}  // end BinarySearch