Flexible schema inclusions and overloading in ShEx.

Hi all.

I would like to point out that ShEx has some ideas about a nice feature
coming from RelaxNG:
"extending and overriding shapes"

cf the discussion here
http://www.w3.org/2001/sw/wiki/Discussion_SHEX_format#Discussion_2.2C_redefining_and_including_other_SHEX_definitions

This is really elegant and handy feature when you deal with data
descriptions on the long term (in standards, for example).

I am not sure SPIN or ICV have any support for an equivalent feature.
True?