Current value of c: NA
Please specify node c content: 
derp
<header>
   <id x_id="1">
      <a />
      <b />
      <c>derp</c>
   </id>
</header>
add content to nodes a & b
<header>
   <id x_id="1">
      <a>Bar</a>
      <b>Foo</b>
      <c>derp</c>
   </id>
</header>