parameters: (self, func)
docstring: Install a callback that receives (name, type, seqid)
          after the message header is read.
docstring_summary: Install a callback that receives (name, type, seqid)
          after the message header is read.
docstring_tokens: Install a callback that receives ( name type seqid ) after the message header is read .
function: def on_message_begin(self, func):
          """
          Install a callback that receives (name, type, seqid)
          after the message header is read.
          """
          pass
function_tokens: def on_message_begin ( self , func ) : pass