docs: function isAssignableFrom(ctor, toTest) {
  
    if (!(ctor instanceof Function) || !(toTest instanceof Function)) {
      throw new Error('arguments must be Functions.');
    }
  
    if (ctor === Object || ctor === toTest) {
      return true;
    }
  
    if (toTest.super_) {
      return isAssignableFrom(ctor, toTest.super_);
    }
  
    return false;
  }