[O] migrating to the snapshot api

[Sam Halliday]

Hi all,

I maintain a fork of an org-mode backend for a markdown based
publishing format
https://github.com/fommil/dotfiles/blob/master/.emacs.d/lisp/ox-leanpub.el

Which works great for the org-mode that ships with emacs 25.2.

However, when I use the latest org-mode from the snapshots on even the
most simple of files, e.g. the one line

==========
Wibble wibble I'm a fish
==========

I get a stacktrace like this (huge amount of data elided)

==========
Debugger entered--Lisp error: (wrong-type-argument sequencep :back-end)
  mapc(#[
  mapc(#[
  org-element-map(
  org-export--footnote-reference-map(
  org-export-collect-footnote-definitions(
  org-leanpub-inner-template(
  org-export-as(leanpub nil nil nil nil)
  org-export-to-buffer(leanpub "*Org LEANPUB Export*" nil nil nil nil
nil (lambda nil (text-mode)))
  org-leanpub-export-as-markdown()
  funcall-interactively(org-leanpub-export-as-markdown)
  call-interactively(org-leanpub-export-as-markdown record nil)
  command-execute(org-leanpub-export-as-markdown record)
  execute-extended-command(nil "org-leanpub-export-as-markdown" nil)
  funcall-interactively(execute-extended-command nil
"org-leanpub-export-as-markdown" nil)
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)
==========

>From reading http://orgmode.org/Changes.html#org1e55804 I can see that
I am supposed to remove the :export-block, which I have done locally.

Can anybody see what else I need to change to be compatible with the
latest version?

Best regards,
Sam