gt;significantly increasing global complexity. E.g. are collections of
&gt;collections the same type of thing as collections of physical objects - or
&gt;do we have a hierarchy of collections. Is the member of group transitive or
&gt;not? (Probably not - but we need to spell this out.)
&gt;It seems to me that spatio-temporal location (or weight) can easily be
&gt;'defined' for sets, but make more sense for sets that tend to have a
&gt;coherent spatio-temporal location - up to a certain granularity (as they
