Syntax: bf(%thread-safe) 

Only used with polymorphic semantic values, and then only required when the
same parser is used in multiple threads: it ensures that each thread's
polymorphic code only accesses the errors counter (i.e., tt(d_nErrors_)) 
of its own parser.

Instead of using the tt(%thread-safe) directive the tt(--thread-safe) option
can also be used.
