The ti(TagTrait) trait class defines, for each of the distinct semantic
values, what the actual data type (tt(DataType)) is that is associated with
the tag and whether the data are mutable or immutable. Mutable data
on the parser's semantic stack may be modified, immutable data may not.

Here is the tt(TagTrait) meta template struct template and its specializations
for the three data types used by our parser.
        verbinclude(//TRAITTAG poly2/sembase/sembase.h)
