title: include multiple files in YAML `in_header:`
body: <p>In an .<code>rmd</code> document that used to compile cleanly with the YAML lines,</p>
  
  <pre><code>output:
    pdf_document:
      fig_caption: yes
      keep_tex: yes
      number_sections: yes
      includes:
          in_header: mystyles.tex spacing.tex
   ...
  </code></pre>
  
  <p>I now get a pandoc error like <code>mystyles.tex spacing.tex cannot be found</code>
  I'm now using R 3.3.2 and R Studio 1.0.153.  I had also tried several other forms (that gave different errors):</p>
  
  <pre><code>        in_header: "mystyles.tex" "spacing.tex"
  
          in_header: mystyles.tex
          in_header: spacing.tex
  </code></pre>
  
  <p>It works if I merge to LateX code in the two files, and just use</p>
  
  <pre><code>        in_header: mystyles.tex
  </code></pre>
  
  <p>What is the syntax for multiple files in an <code>in_header</code> line?  Has it changed?</p>