get a diagram from websequencediagrams.com via API

get-websequencediagrams.el

(require 'deferred)

(deferred:$
  (deferred:url-post "http://www.websequencediagrams.com/" '((message 
"Alice->Bob: Authentication Request
note right of Bob: Bob thinks about it.
Bob-->Alice: Authentication Response
note over A,B: text1
note left of A: text2
note right of A
    multiline
    text
end note
A->B: text
activate B
B-->A: text
deactivate B") (style "default")))
  (deferred:nextc it
    (lambda (buf) 
      (let* ((line (with-current-buffer buf (buffer-string))) url)
        (when (string-match "\\?img=[a-zA-Z0-9]+" line)
          (setq url (concat "http://www.websequencediagrams.com/" (match-string 0 line))))
        (kill-buffer buf)
        (message "GET -> %s" url)
        (deferred:process "wget" "-O" "a.png" url))))
  (deferred:nextc it
    (lambda () (clear-image-cache) (insert-image (create-image (expand-file-name "a.png") 'png))))
  (deferred:watch it
    (lambda () (delete-file "a.png"))))