A cache of which patterns have been processed for a given Path, in
this walk step and the steps that led to it.
A step only adds to it before it makes the caches for the next
steps, so a copy can look up the chain of caches before it, in
place of copying their entries. A pattern for a path that a cache
up the chain has already seen is added to that cache's set, so that
the other steps that share it see it too. Paths are keyed by their
full path, because path-scurry can make a new Path object for the
same path once its cache of children is full.
A cache of which patterns have been processed for a given Path, in this walk step and the steps that led to it.
A step only adds to it before it makes the caches for the next steps, so a copy can look up the chain of caches before it, in place of copying their entries. A pattern for a path that a cache up the chain has already seen is added to that cache's set, so that the other steps that share it see it too. Paths are keyed by their full path, because path-scurry can make a new Path object for the same path once its cache of children is full.