Could someone explain to me why the following is a valid definition of a boost::fusion::set? typedef boost::fusion::set S; S s('a','b','c'); Especially, given that according to description: "set is an Associative Sequence of heteregenous typed data elements. Type identity is used to impose an equivalence relation on keys. The element's type is its key. A set may contain at most one element for each key. Membership testing and element key lookup has constant runtime complexity (see Overloaded Functions). " Src: http://www.boost.org/doc/libs/1_52_0/libs/fusion/doc/html/fusion/container/s... Thanks, Lukasz