+class SmallPtrSetIteratorImpl;
+
+/// SmallPtrSetImpl - This is the common code shared among all the
+/// SmallPtrSet<>'s, which is almost everything. SmallPtrSet has two modes, one
+/// for small and one for large sets.
+///
+/// Small sets use an array of pointers allocated in the SmallPtrSet object,
+/// which is treated as a simple array of pointers. When a pointer is added to
+/// the set, the array is scanned to see if the element already exists, if not
+/// the element is 'pushed back' onto the array. If we run out of space in the
+/// array, we grow into the 'large set' case. SmallSet should be used when the
+/// sets are often small. In this case, no memory allocation is used, and only
+/// light-weight and cache-efficient scanning is used.
+///
+/// Large sets use a classic exponentially-probed hash table. Empty buckets are
+/// represented with an illegal pointer value (-1) to allow null pointers to be
+/// inserted. Tombstones are represented with another illegal pointer value
+/// (-2), to allow deletion. The hash table is resized when the table is 3/4 or
+/// more. When this happens, the table is doubled in size.
+///