///
/// << operator for opening a new subdocument in the core builder.
///
/// @param _
/// An open_document_type token
///
BSONCXX_INLINE key_context<base> operator<<(const open_document_type) {
_core->open_document();
return wrap_document();
}
key_ctx<array_ctx> operator<<(builder::helpers::open_doc_t) {
_concrete->open_doc_append();
return wrap_document();
}
