From a discussion with Petr Rockai: "the possibly non-intuitive bit
that you probably didn't notice is that the hashset is supposed to be
passed to each thread by value. The copy semantics of the entire
hashset are that of a shared pointer: multiple copies share the same
underlying data. Each thread *must* have its own private copy of the
hashset object (there are bits of state that each thread needs for
bookkeeping and those must not be shared)."

*  spot/ltsmin/ltsmin.cc: here.