A Node structure is divided into three parts: linkage, information, and
content.  The linkage part contains memory pointers to the node's
successor and predecessor nodes.  The information part contains the node
type, the priority, and a name pointer.  The content part stores the
actual data structure of interest.  For nodes that require linkage only, a
small MinNode structure is used.

    struct MinNode
    {
        struct MinNode *mln_Succ;
        struct MinNode *mln_Pred;
    };

ln_Succ
    points to the next node in the list (successor).

ln_Pred
    points to the previous node in the list (predecessor).

When a type, priority, or name is required, a full-featured Node structure
is used.

    struct Node
    {
        struct Node *ln_Succ;
        struct Node *ln_Pred;
        UBYTE        ln_Type;
        BYTE         ln_Pri;
        char        *ln_Name;
    };

ln_Type
    defines the type of the node (see <exec/nodes.h> for a list).

ln_Pri
    specifies the priority of the node  (+127 (highest) to -128
    (lowest)).

ln_Name
    points to a printable name for the node (a NULL-terminated
    string).

The Node and MinNode structures are often incorporated into larger
structures, so groups of the larger structures can easily be linked
together.   For example, the Exec Interrupt structure is defined as
follows:

    struct Interrupt
    {
        struct Node is_Node;
        APTR        is_Data;
        VOID        (*is_Code)();
    };

Here the is_Data and is_Code fields represent the useful content of the
node.  Because the Interrupt structure begins with a Node structure, it
may be passed to any of the Exec List manipulation functions.

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