Bubble up / inherit layout vars

_html-layout.html

<ul class="breadcrumbs">
  <li><a href="/">Home</a></li>
  {layout:breadcrumbs}
</ul>

_single-entry-layout.html

{layout="layouts/_html-layout"}

{layout:set name='breadcrumbs'}
  <li><a href="/{segment_1}">Channel Title</a></li>
  {layout:breadcrumbs}
{/layout:set}

template.html

{layout='layouts/_single-entry-layout'}

{layout:set name='breadcrumbs'}
  <li>Entry Title</li>
{/layout:set}