Sense calculation scratch-pad globals. Many of the sense calculations involve recursive descents of portions of the containment tree. In the course of these calculations, it's sometimes useful to have information about the entire operation in one of the recursive calls. We could pass the information around as extra parameters, but that adds overhead, and performance is critical in the sense routines (because they tend to get invoked a *lot*). To reduce the overhead, particularly for information that's not needed very often, we stuff some information into this global object rather than passing it around through parameters.

Note that this object is transient because this information is useful only during the course of a single tree traversal, and so doesn't need to be saved or undone.

transient senseTmp :   object