Advantages:

• 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 object interaction.
• 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.