code: func Required(names ...string) {
  	var at *design.AttributeDefinition
  
  	switch def := dslengine.CurrentDefinition().(type) {
  	case *design.AttributeDefinition:
  		at = def
  	case *design.MediaTypeDefinition:
  		at = def.AttributeDefinition
  	default:
  		dslengine.IncompatibleDSL()
  		return
  	}
  
  	if at.Type != nil && at.Type.Kind() != design.ObjectKind {
  		incompatibleAttributeType("required", at.Type.Name(), "an object")
  	} else {
  		if at.Validation == nil {
  			at.Validation = &dslengine.ValidationDefinition{}
  		}
  		at.Validation.AddRequired(names)
  	}
  }
docs: // Required can be used in: Attributes, Headers, Payload, Type, Params
  //
  // Required adds a "required" validation to the attribute.
  // See http://json-schema.org/latest/json-schema-validation.html#anchor61.