duplicat initial members -> erase_key postcond. fail

[cppljevans]

has_key<erase_key<set<T,T>,T>::type>type::value==true
in violation of Postcondition on
http://www.boost.org/libs/mpl/doc/refmanual/erase-key.html .

[cppljevans]

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The attached file has assoc_item template showing how to check for duplicates
using a boost concept template.  Although it can only be compiled with
Doug's variadic template compiler, the method used to check for duplicates
can, AFAICT, be used with the current m_item and s_item templates in
the current mpl library.  This is because the method used just
uses a slight variation of the static value_by_key which the
current mpl assoc containers already use. 
File Added: cons_inherit_check_test.cpp

[cppljevans@…]

solution using superclass which is fold over insert op