`PreBasis` is a tree, but does not implement the TypeTree interface

I just observed, that, althought a PreBasis forms a tree, it does nt derive from any of the dune-typetree nodes. Instead it defines internally the Node typedef, which is a node around an other PreBasis (in the case of nested structures.)

I seems that this construction makes a somewhat harder to apply tree transformations to a PreBasis. Is there a articular reason? I stumbled over this while trying to implement a typetree conversion from a PreBasis to the appropriate ISTL vector type in PDELab.