undocumented syntax?

I came across this in api.rubyonrails.org and am wondering if it is a
typo or an undocumented feature. Can someone explain?

<% form_tag '/posts' do -%>
    <div><%= submit_tag 'Save' %></div>
  <% end -%>

In particular, I refer to the closing Ruby delimiter '-%>' syntax that
is repeated twice in this example.

Pardon me if this is obvious. I am still new to Ruby.

the - supresses white space output for that ERB line

For more ERB tags, check out: http://stdlib.rubyonrails.org/libdoc/erb/rdoc/classes/ERB.html