code: function subOf(child, mixin) {
    if(child === mixin) return true;
    if(child && child.constructors) {
      for(var i in child.constructors) {
        var parent = child.constructors[i];
  
        // avoid dead loop
        if(parent === child) continue;
  
        if(parent === mixin) return true;
        if(subOf(parent, mixin)) return true;
      }
    }
    return false;
  }
docs: check whether mixin is in inherited chain of child class