Name: envelope
Type: request
Purpose: >
  Process message(s) enclosed in an 'envelope' message.
Response: envelope
Inherits: message
Elements:
  Content: [ required: true, type: string, min_length: 3, max_length: 32768 ]
Notes: >
  The 'Content' element contains a string representing a single message 
  or multiple messages separated by '---' on a line of its own.

  The response envelope will contain the responses to the individual
  messages from the request envelope (in the same order).

  If the request's 'Content' element is encrypted the response's
  'Content' element will be encrypted as well.