sub-string hotspotting, for final approval

[Lofton Henderson <lofton@rockynet.com>]

It turned out to be a little messier than we thought, but here is the final 
take (hopefully).  Stuart, please check that it says what we intended.

FYI, the messiness arose because of the recursion around the <spanned text> 
element -- conceptually, it makes the definition of sub-string APS 
impossible to achieve.  And also our original take had overlooked to allow 
the definition of an APS around an interior sub-string (doh!).

This replaces the recursion with an equivalent non-recursion one, and 
(nicely) the flattened EBNF is identical to the flattened EBNF of the 
original CGM:1999 in the case of no sub-string APS.

I will send this to Dick Puk in the morning, if no one sees any errors.

-Lofton.

substring-APS-defect-minimal.rtf