PRAGMA FUSE_DEF opt[':'cexpr0] cexprm '{'
          tag '=' cexprb
Defines symbolic fuse bits so the end user needn't twiddle them directly.

opt -- a string presented to the user
[:cexpr0] -- which config word stores this entry, starting with 0
cexprm -- the fuse word is bit-wise ANDed with this before continuing
tag -- the sub-tag
cexprb -- which bit to set

These are used by the end user with the `PRAGMA TARGET opt tags' defined above. In this case the result is similar to:

        _config = (_config & cexprm) | cexprb