- problem with passing a function by const reference, but
  a temporary getting created due to the poly coercion, and
  thus a C++ warning about binding a temporary to a reference.
  (This happened in dijstra.g)
- typedef's in concept definitions, typedef's in function headers
- better syntax errors
- find cause of stack overflow for adjacency_list_test.g when
  the vertex_descriptor == int is removed.
- name mangle data members to avoid this problem:
   c++ -Wno-long-double -I./ -I/u/jsiek/boost -I./gc6.2/include -g -c bgl/queue.cpp -o bgl/queue.o
   bgl/queue.cpp:2797: error: declaration of `list priority_queue::list'
   bgl/queue.cpp:1076: error: changes meaning of `list' from `struct list'
   make: *** [bgl/queue.o] Error 1
- public/private in model definitions (instead of "open")
- add capability to open a model: using EqualityComparable<edge>;
- better error messages for when there is no matching overloads, but
   some overload is a close match, perhaps only off by a !.
   * I've improved this somewhat
   - order the overloads based on how close of a match they were
- In some places, it complains about binding temporary to reference, even when
  the type is completely wrong.
- support for enums
- problem with intializing a T* with nullptr (answer, just default construct)
- check timestamp on .g and .fwd files.
- user-defined conversions and Convertible concept
- make sure all paths contain a return statement (unless the return type is void)
- Example of the address-of problem:

    void blah<T>(T x) {
      blah(&x);
    }

- mutable fields in a struct
- const_cast