This method does not do its own synchronization; you should control
thread issues from somewhere else, preferably by synchronizing on the
underlying Map.
There is a big problem with this class; namely, its Headings are stored
in two different structures, neither of which is really definitive, and
both of which are used.
If this parameter is set true, FacetMap will avoid the situation
where there's only one possible next step in any dimension, by
automatically selecting that next step and presenting the options
from there.