[DAS] comments - hierarchical features

[Andy Jenkinson]

Gregg Helt wrote:
> Some may be wondering why not avoid link elements altogether and just 
> represent feature hierarchies by allowing nested feature elements?  So 
> the A1 parent, B1 & B2 part relationship would look like:
> 
> <FEATURE id="A1" ...> ...
>         <FEATURE id="B1" ...> ... </FEATURE>
>         <FEATURE id="B2" ...> ... </FEATURE>
> </FEATURE>
> 
> We considered this during development of DAS/2, but the main use case 
> that argues against it is when multiple parents share children.  For 
> example if multiple transcripts share exons -- this is how alternative 
> splicing is modeled in many GMOD databases.

I think this model is quite common for other databases too.