msc {
  a  [ label = "Alice" ],
  aa [ label = "Alice Server" ],
  bb [ label = "Bob Server" ],
  b  [ label = "Bob" ];

  a  -> aa [ label = "follow @bob@example.com" ] ;
  aa -> bb [ label = "POST Follow" ] ;
  aa <- bb [ label = "Ok 200" ] ;
  bb -> b  [ label = "wait for approval" ];
  ...;
  b  -> bb [ label = "approve" ];
  bb -> aa [ label = "POST Accept" ] ;
  bb <- aa [ label = "Ok 200" ] ;
}
