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            Returns a type convertible to bool
            that evaluates to true if
            the sequence contains an element associated with a Key, else, evaluates
            to false.
          
template <typename Key, typename Sequence>
typename result_of::has_key<Sequence, Key>::type
has_key(Sequence const& seq);
| Parameter | Requirement | Description | 
|---|---|---|
| 
                       | Model of Associative Sequence | The sequence we wish to investigate. | 
| 
                       | Any type | The queried key. | 
has_key<Key>(seq);
            Return type: Convertible to bool.
          
            Semantics: Evaluates to true if the sequence contains an element
            associated with Key, else, evaluates to false.
          
#include <boost/fusion/sequence/intrinsic/has_key.hpp> #include <boost/fusion/include/has_key.hpp>
set<int, char, bool> s(1, 'x', true);
assert(has_key<char>(s) == true);