RPC

    Here is suggestion to use KEKS codec with a simple client-server
    Remote Procedure Call interface. It is very similar to
    => JSON-RPC
    => MessagePack-RPC

    Call, reply and notify message headers ([schema/tcl] format):

        call {
            {field . {list} len=4}
            {field 0 {str} =c}
            {field 1 {hexlet}} {# id}
            {field 2 {str} >0} {# method}
            {field 3 {map}}    {# params}
        }

        reply {
            {field . {list} len=4}
            {field 0 {str} =r}
            {field 1 {hexlet}} {# id}
            {field 2 {str}}    {# error}
            {field 3 {map}}    {# body}
        }

        notify {
            {field . {list} len=4}
            {field 0 {str} =n}
            {field 1 {hexlet}} {# id}
            {field 2 {str} >0} {# method}
            {field 3 {map}}    {# body}
        }

    It is advisable to use UUIDv7 for call/notify's id.
    Reply's id must be the same as in corresponding call.
    Empty error string means no error occurred.

    Notification differs only by its type and no reply expectation.