lib/hbs.js
function parseLayout(str, filename, cb) {
    var layoutFile = self.declaredLayoutFile(str, filename);
    if (layoutFile) {
      self.cacheLayout(layoutFile, options.cache, cb);
    } else {
      cb(null, null);
    }
  }
function parseLayout(str, filename, cb) {
    var layoutFile = self.declaredLayoutFile(str, filename);
    if (layoutFile) {
      self.cacheLayout(layoutFile, options.cache, cb);
    } else {
      cb(null, null);
    }
  }
function parseLayout ( str , filename , cb ) { var layoutFile = self . declaredLayoutFile ( str , filename ) ; if ( layoutFile ) { self . cacheLayout ( layoutFile , options . cache , cb ) ; } else { cb ( null , null ) ; } }
Allow a layout to be declared as a handlebars comment to remain spec
compatible with handlebars.

Valid directives

{{!< foo}}                      # foo.hbs in same directory as template
{{!< ../layouts/default}}       # default.hbs in parent layout directory
{{!< ../layouts/default.html}}  # default.html in parent layout directory
Allow a layout to be declared as a handlebars comment to remain spec compatible with handlebars .