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public:
Function for list stream output.  Takes O(n) time.
   friend Writer&   operator << (Writer& w,  const List<X>& list)
Function for list stream input.  Takes O(n) time.
   friend Reader&   operator >> (Reader& r, List<X>& list)
Function for testing that list is in a valid state.  Takes O(n) time.
   friend void OK(const List<X>* list)
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