On: http://www.boost.org/libs/mpl/doc/refmanual/map.html there's the following row in the "Expression Semantics" table: insert<m,x>::type A new map equivalent to m except that at< t, key_type<m,x>::type >::type is identical to value_type<m,x>::type. which is confusing because the 't' in at< t, key_type<m,x>::type >::type doesn't occur in the 1st column of that row. Should it read: insert<m,x>::type A new map, t, equivalent to m except that at< t, key_type<x>::type >::type is identical to value_type<x>::type. ? I'm assuming that x is an mpl::pair<k,v>, for some key, k, and value, v.