Make sure we don't get duplicate reference links, even with `--reference-location=section`. ``` % pandoc --reference-links -t markdown --reference-location=section --atx-headers # a ![](a) # b ![](b) ^D # a ![][1] [1]: a # b ![][2] [2]: b ``` Subsidiary issue: allow line break between reference link url/title and attributes: ``` % pandoc [a] [a]: url {.class} ^D <p><a href="url" class="class">a</a></p> ```