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- The ability to define a coarse boundary simplified much of the
reasoning, because overall relationships are quickly lost when
traversing a boundary which consists of a chain of fine details with
imprecise relationships between them. The coarse boundary description
was used more often than the fine one for most operations.
- The definition of proximity as boundary elements that ``might contact
each other'' provided explicit information useful for reasoning about
- The partial distance ordering implicitly supports graceful degradation
with inexact information, and the reasoning system based on it is
naturally robust as a result.
- The doubly-linked circular list formed by boundary left and right pointers
was a useful aid to boundary operations. The resulting boundary
representation is similar to that used by Faltings in the CLOCK system
[FNF87], but is more closely related to actual shape
than that system, which explicitly links ``first'' and ``last''
elements of the boundary.
- An EPB/PDO representation could be constructed directly from the output of
vision systems such as those described by Brady, and by MacKerras.