+Structure of a MIME message is tree.
+<p>
+In the tree, root node is the entity indicates all of the message. In
+this document, it is called <concept>root-entity</concept> or
+<concept>message</concept>. In FLIM, it is indicated by buffer local
+variable <code>mime-message-structure</code>.
+<p>
+Each entity except root-entity has a parent. An entity may have
+children. We can indicate an entity by relative position from a base
+entity, based on the parent-child relationship.
+<p>
+In addition, we can indicate an entity by absolute position of the
+message.
+<p>
+Each entity, which is a node of the tree, can be numbered by
+depth and left-to-right order of the depth.
+<verb>
+
+ +-------+
+ | nil |
+ +---+---+
+ +-------------------+-------------------+
+ +-+-+ +-+-+ +-+-+
+ | 0 | | 1 | | 2 |
+ +-+-+ +-+-+ +-+-+
+ | +---------+---------+ |
+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+
+ | 0.0 | | 1.0 | | 1.1 | | 1.2 | | 2.0 |
+ +-----+ +-----+ +-----+ +-----+ +-----+
+
+</verb>
+<p>
+Namely, if depth of a node is n, the node has a node-number, which is
+consists of n integers. In this document, it is called
+<concept>entity-number</concept>. An entity-number is represented by
+list of integer, like <code>(1 2 3)</code>.
+<p>
+mime-entity has also <concept>node-id</concept>. A node-id is
+represented by reversed list of entity-number. For example, node-id
+corresponding with 1.2.3 is <code>(3 2 1)</code>.
+<p>
+Each entity can be indicated by entity-number or node-id in
+<code>mime-message-structure</code>.
+