SPARQL prefixes and local parts

The current SPARQL specification states the following for prefixed names:

The PREFIX keyword associates a prefix label with an IRI. A prefixed name 
is a prefix label and a local part, separated by a colon ":". It is mapped 
to an IRI by concatenating the local part to the IRI corresponding to the 
prefix. The prefix label may be the empty string.

The grammar for local parts does not allow slashes in the name.

I would like to suggest that local parts should in fact allow for slashes 
in the local part of the name.   This allows one to define prefixes that 
are basically the root of some structure. 

For example, one might wish to define a prefix

PREFIX ex1:  <http://www.example.org/schemas/Concept1>
PREFIX ex2:  <http://www.example.org/schemas/Concept1>

and therefore specify abbreviated IRIs such as:

ex1:SubConceptA/SubConcept1

and 

ex2:SubConceptB/SubConcept2

Without this ability, one must either define a very flat namespace, or 
define separate prefixes for every concept.

-Mark Wood