anchor.diff:  
   ==== Header: *id*
   
  -The `id` header serves for identify messages. It is mainly used by client frames. Frames
  -without specified `id` does not return responses. And some frames has `id` as mandatory
  -header.
  +The `id` header serves for identify messages. It is mainly used by client frames.
  +Frames without specified `id` does not return responses. And some frames has `id`
  +as mandatory header.
   
   
   ==== Header: *content-type*
  
positive.code: ----
  
  
  === ACK
  
  Is a generic frame that is used for acknowledge the messages received by the server.
  It represents the signal that the server have recived a message. Is a body less
  version of `RESPONSE` frame.
  
  This is aspect of the `ACK` frame:
  
  [source, text]
  ----
  ACK
  message-id: e11a27d3-4b86-4191-a310-12847fa79d48
  
  ^@
  ----
  
  The `ACK` frames should mandatory identify the associated message. So if a message
  sent to the server does not provide any identification id, the `ACK` frame should
  not to be send by server.
  
  
  === ERROR
  
  Is a generic frame that represents a error situation. This kind of messages can be send
  by server when something goes wrong.
  
  This is aspect of the `ERROR` frame:
  
  
negative.code: 
  === HELLO
  
  A POSTAL client initializes the communication with the server sending a `HELLO`
  frame:
  
  [source, text]
  ----
  