R-Tree operators

I don't understand what overlap_{left,right} are supposed to mean.

In general what seems to be missing from the documentation of operator
strategies and support functions is what invariants between them are required.

Can I just make up a meaning for overlap_{left,right}? That's what other
r-tree indexable data types seem to do. Does it have to be consistent in some
with the definitions for left/right? What about with the definitions for
overlapping? contains/contained by? What does it mean to be consistent with
them?

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greg