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boost::proto::when<Grammar, >::impl
// In header: <boost/proto/transform/when.hpp> template<typename Expr, typename State, typename Data> struct impl : proto::result_of::has_env_var<proto::transforms_type>::proto::result_of::env_var<proto::transforms_type>, proto::result_of::env_var<proto::data_type> >:: { };
The implementation of the impl
struct depends on whether the Data
parameter is a transform environment that contains a value corresponding to the
proto::transforms_type key. If so, that value is treated as a
map from rules to transforms. Otherwise, the Data
type itself is treated
as such a map.