[Boost-users] Inserting elements in extensible associative sequences

The semantics of MPL's insert metafunction for an extensible associative sequence is defined as: "For any Extensible Associative Sequence s, iterator pos in s, and arbitrary type x: typedef insert<s,x>::type r; Semantics: r is concept-identical and equivalent to s, except that at< r, key_type<s,x>::type >::type is identical to value_type<s,x>::type." What happens if the key type of x is already in s? Thanks, Eric.