SVTYPEMASK curiosity.

I just changed SVTYPEMASK to 0x3F, and make test'd it -
everything worked.
Is there a compelling reason to reserve the top 3 bits (we
now have 16 
svtypes,
so even 5 bits gives us headroom for 16 more types)


Or conversely, is there anything to be gained with 3 more
sv-flags ?