Subtyping: do we need to return a single node or a list of nodes from the rule? Follow

Hi,

I'm experimenting with the MPS type system and I'm a bit confused by subtyping rules at the moment. As far as I could see in the baseLanguage.mpr , the subtyping rule

subtyping rule int_extends_long {                
   weak = false                                  
   applicable for concept = IntegerType as intType
                                                 
   rule {                                        
      return  < long > ;                         
   }                                             
}                                               

resulted in the following generated code:

public class int_extends_long_SubtypingRule extends SubtypingRule_Runtime implements ISubtypingRule_Runtime {

  ...

  public SNode getSubOrSuperType(SNode intType) {
    return new _Quotations.QuotationClass_49().createNode();
  }

Thus, it seems to be possible to return a single node containing type information from the rule body. On the other side, when I've tried to build a similar construct (as a sample) in my own test project:

subtyping rule integerExtendsNumeric {               
   weak = false                                      
   applicable for concept = TypeInteger as typeInteger
                                                     
   rule {                                            
      return  < numeric > ;                          
   }                                                 
}               

I've got the following code:

public class integerExtendsNumeric_SubtypingRule extends SubtypingRule_Runtime implements ISubtypingRule_Runtime {

  ...

  public List<SNode> getSubOrSuperTypes(SNode typeInteger) {
    return new _Quotations.QuotationClass_6().createNode();
  }

which, obviouly, failed to compile.

Of course, it is not a problem to build a list of nodes with a single element and return it from the rule as a workaround, but I'm not sure why generated code was different.

Could you shed a light on this, please?