{
    "states": [
      "AUTHORIZATION",
      "TRANSACTION",
      "UPDATE"
    ],
    "initial_state": "AUTHORIZATION",
    "final_states": [
      "UPDATE"
    ],
    "transitions": [
        {
            "from": "AUTHORIZATION",
            "event": "send greeting",
            "action": "reply +OK POP3 server ready",
            "to": "AUTHORIZATION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "receive USER",
            "action": "reply request password",
            "to": "AUTHORIZATION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "receive PASS",
            "action": "cond check password",
            "to": "AUTHORIZATION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "cond valid USER/PASS",
            "action": "set authenticated true",
            "to": "TRANSACTION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "receive APOP",
            "action": "cond check digest",
            "to": "AUTHORIZATION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "cond valid APOP",
            "action": "set authenticated true",
            "to": "TRANSACTION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "cond open maildrop failed",
            "action": "reply -ERR and release lock",
            "to": "AUTHORIZATION"
          },
          {
            "from": "AUTHORIZATION",
            "event": "receive QUIT",
            "action": "reply +OK and close TCP",
            "to": "UPDATE"
          },
          {
            "from": "TRANSACTION",
            "event": "receive STAT",
            "action": "reply message count size",
            "to": "TRANSACTION"
          },
          {
            "from": "TRANSACTION",
            "event": "receive LIST",
            "action": "reply scan listing",
            "to": "TRANSACTION"
          },
          {
            "from": "TRANSACTION",
            "event": "receive RETR",
            "action": "reply full message",
            "to": "TRANSACTION"
          },
          {
            "from": "TRANSACTION",
            "event": "receive DELE",
            "action": "set message deleted",
            "to": "TRANSACTION"
          },
          {
            "from": "TRANSACTION",
            "event": "receive NOOP",
            "action": "reply OK",
            "to": "TRANSACTION"
          },
          {
            "from": "TRANSACTION",
            "event": "receive RSET",
            "action": "reset deleted flags",
            "to": "TRANSACTION"
          },
          {
            "from": "TRANSACTION",
            "event": "receive QUIT",
            "action": "set session closing",
            "to": "UPDATE"
          },
          {
            "from": "UPDATE",
            "event": "cond deleted messages exist",
            "action": "delete marked messages",
            "to": "UPDATE"
          },
          {
            "from": "UPDATE",
            "event": "cond cleanup complete",
            "action": "release lock and close TCP",
            "to": "UPDATE"
          },
          {
            "from": "UPDATE",
            "event": "timeout 5s",
            "action": "close TCP connection",
            "to": "UPDATE"
          }
    ]
  }