code: function each(key, interface, fn) {
    var stack = interface.stack;
  
    for (var i = 0, len = stack.length; i < len; i++) {
      stack[i][key].forEach(fn);
    }
  }
docs: Interate through all functions in the stack.
  
  @param {String} key
  @param {Interface} interface
  @param {Function} fn
  @api private