This division is purely logical -- in the underlying implementation,

the message type can choose to have different means of storing the

data,dependinn on the type used to tag the message. This section

data,depending on the type used to tag the message. This section

covers the `Message Concept`_ as well as the `basic_message`_

implementation.

**Legend**

:M: The message type.

:H: A headers container type.

:m,n: An instance of **M**.

:S: A string type.

:s,k,v: An instance of **S**.

| ``destination(m);``| unspecified| Retrieve the destination of ``m``.|

| ``headers(m);``| ``Range<Pair<S,S>>``| Get the range of headers of ``m``.|

| ``headers(m);``| unspecified| Get the range of headers of ``m``. The|

||| result should be convertible to ``H``|

| ``body(m);``| unspecified| Retrieve the body of ``m``.|

| ``m << source(s);``| ``M &``| Set the source of ``m``.|

| ``source(m,s);``| ``M &``| Set the source of ``m``.|

| ``m << destination(s);``| ``M &``| Set the destination of ``m``.|

| ``destination(m,s);``| ``M &``| Set the destination of ``m``.|

| ``m << header(k, v);``| ``M &``| Add a header to ``m``.|

| ``add_header(m, k, v);``| ``M &``| Add a header to ``m``.|

| ``m << remove_header(k);``| ``M &``| Remove a header from ``m``.|

| ``remove_header(m, k);``| ``M &``| Remove a header from ``m``.|

| ``m << body(s);``| ``M &``| Set the body of ``m``.|

| ``body(m,s);``| ``M &``| Set the body of ``m``.|

Types that model the Message Concept are meant to encapsulate data

that has a source, a destination, one or more named headers, and a