code: function forEachExpansion(expansions, fn) {
      let impl = function(expansions) {
          expansions.forEach((expansion) => {
              fn(expansion);
              impl(expansion.expansions);
          });
      };
  
      impl(expansions);
  }
docs: Recursively iterate over the expansions and
  execute the passed callback against it.
  
  @method forEachExpansion
  @private
  @param {Array<Object>} expansions
  @param {Functin} fn