3.16.6.2 Derivation Valid (Restriction, Simple) appeals several
times to the notion of "valid restriction", e.g. clause 1.3.2 
"... the DF's {value} is a valid restriction of BF's {value} 
as defined in [XML Schema: Datatypes]."

Datatypes does not define the notion of one value being a 
valid restriction of another value in isolation; it defines
a number of constraints called "length valid restriction" and
"whitespace valid restriction" and so on.  The reference
from Structures needs to make clearer what is going on and what
parts of Datatypes are being appealed to.  Datatypes may also
need to define a more useful term for the required relation
on facet/value pairs.